Oh also we live in the northern part of Illinois. Driving will be the more reasonable way to go I think but we only have 7 days and don't want to spend most of that time driving.

Take the "City of New Orleans" to New Orleans. The CONO leaves Chicago at 8PM every night and has several stops in IL on its way south. If you don't like the French Quarter, rent a car and head east on the Gulf Coast. Biloxi has made a big recovery from the hurricane.

What is that??

It's Amtrak

or you can drive to Lorton, VA and take Amtrak Auto Train to Sanford, FL. Your family and your car will get transported to Florida, just outside of Orlando. The fare covers just about everything (dinner, wine, breakfast), except for the private rooms/suites. If you choose not to do a room then expect a very comfy but possibly noisy coach car.

Well, when driving you can reasonably expect to travel a distance of about 50 miles for every hour of driving - assuming there are no weather related delays and no major traffic accidents along your route. Thus, 500 miles will take you about 10 hours and if you drove for 12 to 16 hours you could stretch that distance to between 700 and 800 miles - it makes for a long day but that about a far as you could hope to get before fatigue sets in.

So, first determine how long you are willing to take to reach your intended destination. Since you only have "7 days" I'm guessing you don't want to spend more than a day or 1 1/2 days to get to your destination.

You could reach the coastal regions of North Carolina, South Carolina and many Gulf Coast locations (Florida Panhandle, New Orleans, parts of Texas) in about 15 to 17 hours of drive time and since you have several people in your party of "driving age" you could share the duties and make in by driving straight though in one day.

The thing is, however, even if you were to make it to any of those beach areas there's no guarantee it will be warm enough for swimming at the end of December- in fact the odds are quite small it will be swimming weather.

The aforementioned Autotrain takes about 17 hours to reach Florida but it does travel at night (leaves 4PM, Arrives 9:30AM) so you'd still have to drive about 12 hours to reach Lorton, VA. The train drops yo off in the center of Florida, near Orlando which means another hour or two of driving to reach the nearest beaches.

Go to Google Maps or Mapquest - you can plot out routes from your home to various destinations to determine distances and travel times. That should help you determine what's available within a "reasonable driving distance" (for you) . once you know that you can research things to do in those locations and choose a spot that most interests you and your family.

Amtrak isn't an option unless you can increase your budget. Roundtrip from Chicago to New Orleans is around $300, and you only have $500-600 per person. That leaves you around $40 per person per day for lodging, rental car, food and whatever else you want to do. Just not realistic.

Driving to the Gulf Coast is certainly a good option. You really won't find anywhere in the continental U.S. that has reliably warm beach weather the last week of December. So, what are your other interests? I'd head for somewhere along the coast that interests you, and go to the beach if it's warm enough but don't plan on that.

New Orleans is great but you'd need to get out of town before New Years, I don't think you have room in your budget for what a hotel will cost then. Not only is New Year's Eve expensive, but the Sugar Bowl is on Jan. 2nd and fans of both teams will probably start showing up the weekend before, driving up hotel costs.

Personally I think a road trip south through Memphis, down to New Orleans, then east along the Gulf Coast to maybe Pensacola before heading back home through Alabama, Tennessee and Kentucky is the way to go. You can adjust your route based on what things interest you and weather.

The Amtrak CONO also travels through the night. Arrive in New Orleans mid afternoon and spend the night there. Rent a car the next morning. If you want to go into Texas you can get to Port Isabel in about 13 hours. Across the causeway from Port Isabel is South Padre Island which is about the same latitude as Fort Lauderdale FL (26* N).
The standard fare on the CONO is $125/adult but you avoid a night in a hotel room.
The coach fare on the Auto Train is $100/adult plus at least $175 for transport of the car (one way).

I don't think a beach will be in your possibility. VERY unlikely to be warm and certainly not for swimming.
What does the family like to do--you could go to Chicago and sightsee, but you have probably done that somewhat.
You could go to NOLA and sightsee, driving both ways.
YOu could go to Washington DC/NYC/Cleveland. choose a place and see what is available to do. Nashville/Memphis.

I like the CONO idea if you can get it into your budget. Perhaps take it one way to NO and then rent a car and bring it back, driving up the Mississippi as suggested above and below.

Alternatively, drive over to and down the Mississippi to New Orleans as WhereAreWe suggests. Only problem with this is possible snow. I've read recently that warm winters are alternating with cold, so maybe this December will be nice.

I don't think a train trip is in the budget.

And unfortunately driving to a warm beach from Illinois in December in 7 days simply isn't going to work. It's just too great a distance (say to get down to Florida and back).

How about figuring out something closer to home you could do this time, and save more money so in another year or two you could afford plane tickets or that cruise?

The Carnival Elation is the only cruise ship sailing out of New Orleans in December.
Th warmest beaches in the US in December are in Hawaii.
Warmest in the continental US would be in Texas south of Corpus Christi or in Florida south of Miami.

Just a thought but have you looked at the budget airlines at your nearest airport? Sometimes they fly very cheap flights to Florida, Arizona, or other warm places. I'd suggest going wherever you can get a cheap flight.

The flight is a good idea but unfortunately they will be arriving in HIGH HIGH (read "expensive") season for lodging. And it IS eyewatering!!
closer to home--make it fun--go out to a great dinner splurge.

The responders to this thread and the similar one OP posted on the "Travel Tips & Trip Ideas" forum have tried being helpful but the reality is OP is planning to travel during Christmas/New Years Week (perhaps one of the most expensive times of the year) on budget for 5 grown (or almost grown) people that is unrealistic for the time period and length of the trip.

Even if OP used "budget airlines" the fare (say even $125 pp each way) would use up a almost half the "maximum" per person budget of $600. And driving anywhere that takes more than a day will eat up a lot of valuable time.

OP should heed the general advise given by suze (and hinted at by others) and look at things to do closer to home or forego any real trip this year and continue to save until they can afford that "dream cruise".

Sometimes leisure travel just isn't in the cards. I wish OP good luck in reaching a proper decision.

Another option for a cruise -- instead of driving south, drive EAST and take a cruise out of Baltimore or one of the New York ports. It may take you about 10-12 hours for the drive vs longer to other southern destinations listed above.
Baltimore only has the 7 day cruises; the others are 9-10 days.
For example,
Carnival has a 7 day from Baltimore MD (easy port to access and if you stay overnight at some local hotels, you can leave your car there during your cruise instead of at the terminal) that goes to Port Canaveral FL and the Bahamas.

You can do an easy search on vacationstogo.com by month/location/line/price, etc. to see what will work for you.

Unfortunately, a quick search of the Carnival website shows the lowest per person rate for their December 22 departure out of Baltimore is $829 which is 38% more than upper limit of OP's $600 per person budget.

It's also a 7 day cruise and OP only has 7 days for their entire vacation (OP has not given specific dates - stating only "last week in December") presumably they don't have extra time to travel to/from Baltimore.

They don't want to drive a long ways and they don't have enough funds for any kind of a cruise. Please let's give realistic advice here. $2500-3000 simply isn't enough to get 5 people from Illinois to a warm beach in December. That budget breaks down to only $85 per day per person.

I know the weather isn't what you hoped for but maybe renting a cabin somewhere within a few hours drive of your home?

Lay off the focus on a beach. OP said they want warmer weather - so that's anywhere south of them. OP said possibly a beach but they wanted more to do than just lay around on a beach.

Obviously with their budget they can only do a road trip somewhere within a days drive. Train and plane is out of the question.

Like I said before, head south to the Gulf Coast. Make a road trip out of it instead of hauling ass to get somewhere. If its warm enough to go to the beach for a little bit, great. If not, there's plenty of other things to do along the way.

It's a bad time to find budget lodging in bigger cities, but you can find reasonable rates in smaller cities.

The cheapest alternative for a warm weather getaway from Illinois would be to drive nonstop (3 or 4 drivers) to South Padre Island Texas. You can drive there from nearly all of Illinois in 24 hours with fuel costs in the $180 range.
The Hilton Garden Inn on SPI has suites with 2 Queen beds at a reasonable rate that would fit the budget.
If there is any money left in the budget on the way back see either Dallas or St. Louis.

nearly all of Illinois in 24 hours with fuel costs in the $180 range.

how did you figure that? It's about 1400 miles from Chicago. Let's assume the car gets about 20 mpg and assuming that gasoline is ~$4 per gallon, it will be about $280 one way just for gas.

1440 miles @ 60mph @ 32 mpg = 45 gallons each way. My Honda Civic always does better than that. 45 gallons at $4 = $180.
For more than 2 people in a car, driving costs are less than the same number of people on a train.

Folks give it up. If not a troll==good post, you have to say. It isn't real.
Illinois=far north
beach=far south--AND warm
Not happening=true

Allegiant Airlines has incredible fares from smaller one off cities to Florida and Arizona.

As others have mentioned - I think a warm-weather destination is going to be out of reach, given that you don't want to spend a lot of time driving and train/plane fares will likely bust your budget. I agree with suze. Forget about warm and think cozy instead, like a lodge with a fireplace, etc.