I know you've made up your mind to fly, and I definitely second (and third and fourth!) that recommendation, mostly because it will give you more time in Chicago. I grew up in NY and have lived in San Francisco for many years, and Chicago is one of my very favorite cities in the world!

Wildblueyonder, glad you're so excited to see Chicago. I agree that you'll be here during a great time of year. In addition to the the museums, architecture cruises, festivals and lakefront parks you may be able to catch the Grant Park Symphony. They play free concerts throughout the summer in the Pritzker Pavillion in Millennium Park.

There are also free movies in Grant Park. You can take snacks and watch such classics as The Sound of Music or Double Indemnity. Finally, Navy Pier has fireworks throughout the summer on Wednesdays and Saturdays. For a complete list of events when you'll be here, check www.metromix.com or www.chicagoreader.com.

They play free concerts throughout the summer in the Pritzker Pavillion in Millennium Park.

Even without the music, the Pritzker Pavillion (and all of Millennium Park) is spectacular!

Thanks again for all responses, suggestions/ links. I'm getting really excited about Chicago and agree that the extra time will be VERRRRRY useful. I hope that the Marriott will let us dump our luggage with them, as you can't book in until 3pm. And another bonus - we'll have to change our car hire - and I looked for the Alamo place in Chicago and it's about 2 feet from the hotel! Slight exaggeration, maybe - but it's on North La Salle, just behind W Wacker Drive!

Just to update - I've now booked our flight (JetBlue 9:45am) from JFK to Chicago and changed our accommodation/car hire etc. Our full itinerary is now as follows. Thanks again to all who helped with the latest decision!! I know that this itinerary is a bit crowded and we should spend more time at all of the places, but we only have this amount of time to play with.

Day 1 - Heathrow to JFK - courtesy car (lots of luggage for 3) to Affinia Dumont, Manhattan.
Day 2 - free in NYC
Day 3 - free in NYC
Day 4 - leave Manhattan by 7 (!) and return to JFK for flight to Chicago - taxi/courtesy car to Marriott Rennaissance on Wacker Drive.
Day 5 - free in Chicago
Day 6 - pick up hire car (from Alamo, 2 blocks from hotel) - drive to La Crosse, via Galena, Dubuque, great Mississippi river road. Overnight Holiday Inn (other hotels are fully booked)
Day 7 - Across Minnesota and into South Dakota, stopping at Blue Earth (Jolly Green Giant!!), Sioux Falls (hey I'm a geologist I want to see the pink quartzite in Falls Park!) and Mitchell for the Corn Palace. Overnight at the Americinn, Chamberlain (on Missouri).
Day 8 - Badlands and Wall Drug Store, before continuing to Hill City for 3 nights in cabin.
Day 9 - free in Black Hills
Day 10 - free in Black Hills
Day 11 - via Hot Springs, cut corner of Nebraska (tick off another state! , into Wyoming (been there before, so no new tick!), and into Colorado (tick!) and overnight at the Homewood Suites, Fort Collins.
Day 12 - Through Rocky Mountain NP and down to Vail (area) for an overnight in a condo (Lakeside Terrace) in Avon.
Day 13 - To Moab and Arches NP. Overnight at Red Cliffs Lodge.
Day 14 - To Monument Valley - overnight at Gouldings Lodge
Day 15 - Hope to have enough time to make a quick trip to North Rim GC (been to the South Rim), before continuing to Zion (been to Bryce) - been to Zion before, but didn't have enough time for anything much. 2 nights in Flanigan's (stayed in the Desert Pearl last time and it was lovely - but we wanted to try another hotel).
Day 16 - Free in Zion
Day 17 - to Las Vegas, via Lake Mead/Hoover Dam (missed this on last trip) - overnight at the Bellagio (woo!)
Day 18 - can't wait to get out of Vegas... Up to Death Valley (it will be HOT - but must be done!) Overnight at Lone Pine, Dow Villa Motel.
Day 19 - To Palm Springs. We KNOW it will be very hot, but we've booked a nice little house with its own pool/spa.
Day 20 - free in Palm Springs
Day 21 - free in Palm Springs
Day 22 - to San Diego. 2 nights at the Dana, Mission Bay.
Day 23 - free in San Diego
Day 24 - our flight back to London, from LAX, isn't until 9:10pm, so we will have all day to drive up the coast road, stopping here and there.
Day 25 - arrive back at Heathrow.

Okay, I know you have decided, but here is more assurance that flying is the way to go. Having done this drive more times than I care to count, I honestly get a nervous twitch just thinking of the boring miles spent in Indiana, Ohio and PA.

Your trip looks great by the way. One small suggestion, I notice day 11 is in Fort Collins. I highly recommend the Armstrong as a place to stay, it is located in historic downtown with plenty of charm and fun things to do. Although I also notice you aren't planning to stay there long, so not sure if you want to change.

Just in case...here is the link.
http://tinyurl.com/2x7poy

259 S College Ave
Fort Collins, CO 80524
(970) 484-3883

Piling on: fly.

We've done this trip over the years a few times, and it never gets better. (We'd drive it when we intended to spend almost a month visiting in the East, just to have our car.)

While there are various sites of some interest, I don't think these would be your priorities for the time you have. You'll go a little nuts without satellite radio or books-on-CD -- radio reception is not good on many parts of the long highway through PA.

Depending on specific days you'd like to fly, many of the airlines should have some good prices between New York and Chicago. Personally, I'd prefer to spend an extra day in New York, or almost anywhere else on your trip.

Re: your question about extra charges for 2 checked bags. United Airlines recently announced a surcharge of $25 (I think) for more than one checked bag if traveling in coach class. So far, I think the other major airlines (American, Delta, etc.) do not have this surcharge.

It may be too late. But I would strongly, strongly, strongly in June or any other time, fly Southwest to Midway.

All the time envolves gets fractualized. You will have much more time in Chicago.

You can take a much quicker orange line from MWY to downtown Chicago and you flight will feel 1/2ed of what you experience going through and from OHare.

StephCar - thanks for the suggestion of the Armstrong. It looks lovely - but we are only 'passing through' Fort Collins and I think we'll be fine in the Homewood Suites. Priority for us is a bed each (being 3 of us!), even if one of those beds is a sofa bed!

JJ5 - yes, too late, I've got the etickets for the JetBlue flights, from JFK to O'Hare. Southwest only flies from Long Island, which would be a bit inconvenient from Manhattan? We're planning on taking the Go shuttle from O'hare to downtown - has anybody used them? We *should* arrive at Chicago at around 11:30am and I don't know what on earth we're going to do (apart from the trip into downtown) from then as we'll be lumbered with luggage and not able to check into our room (Renaissance) until 3:00pm!

There's a pretty good chance you will be able to check-in early (I checked into a Chicago hotel at 10:30 a.m. last weekend, and the hotel was almost fully booked.) Otherwise, the hotel will be able to hold your bags for you, and you can either get something to eat or walk around. The Renaissance is in a great location to get to Michigan Ave and Millennium Park, and there's a lot of restaurant options of all price ranges within either walking distance, an inexpensive cab ride, or a quick trip on the El.