We are coming from upstate NY and looking at flying out of Newark Airport in late September on a Friday around noon to Las Vegas and returning to Newark on the following Tuesday around 4pm. At that point we'd like to travel out to the end of Long Island for a few days. The thought was to drive down to Newark Thursday night and park at a hotel and fly out Friday morning and drive to Long Island on return. I've read the posts about hotels in the airport area and can find one, but on return, it seems like the traffic from Newark to Long Island that time of day will be awful. Should we wait until later in the day to leave Newark and get in on a later flight? Or should we start out Thursday night somewhere on the Long Island side and take public transportation to and from Newark so when we return do can take public transportation back to the car and avoid the traffic?

I would not put public transportation into the mix just to avoid Friday traffic. You would be adding a different kind of agony and expense to your travel.

Keep the flights you have. You have no idea if your flight will arrive on time and how long it will take you to retrieve your car and get out of Newark Airport. What if the later flight is delayed? Then you'd have to drive to Long Island after a very long day, which might be more problematic than dealing with rush hour traffic.

If you're really concerned about rush hour, once you've retrieved your car and see the time, stop somewhere in the area for dinner and head to Long Island a bit later.

For your departure Fri noon it really doesn't matter where you stay. There is no need to be very close to the airport - you could just as easily find a convenient hotel in northern NJ or southern NY along your route. If you are driving the NYS Thruway o Rt17 from upstate NY you could grab a hotel around Exit 15 near Mahwah NJ. Just google 'hotel near Mahwah NJ' and you'll see a list.

As for Tue return, I would definitely wait until later to try to avoid the traffic in and through NYC. No way would I try to cross from NJ to NY and drive through NYC at 5pm on any of the possible routes.

Public transportation from Newark to/fro Long Island would be helishly convoluted and time-consuming - involving a couple of different railroads and transferring at very busy times.

Also - nearby LI is NOT noted for good places to stay. You're better off staying near Newark (the hotels there are at least as good/cheap as those on nearby LI).

On your return you will hit the middle or rush hour. I would wait until about 8 pm to leave for LI (pick up you luggage, get your car and go have dinner somewhere) or stay another night at Newark and leave for LI at 6 am (but face possible commuter traffic). Expect the trip from Newark to Montauk (?) to be about 3.5 hours (if you leave directly when you land and get caught in traffic it may be close to 5 or 5.5 hours).

Well, I haven't actually booked the flight yet, and now I see there is an airport ISP in Long Island that has direct flights to Las Vegas at reasonable rates. Would this airport be the better option maybe?

If your destination is Long Island, then yes, flying in and out of Long Island makes a lot of sense. If the flight is on Southwest be sure to check if it is a non stop flight. It might be a direct flight, but that can often mean 1-2 stops enroute on SW.

You can also fly SW from Albany to Las Vegas, so an option is to fly out of Albany, into Islip, then rent a car one way back to Albany.