Thanks for the comments. More food for thought. Although I must admit that NYC and DC are probably what I'd be looking forward to the most (political junkie )

My opinion differs. I've driven across this county multiple times and I think 3 weeks is plenty of time to see a lot of different places if you are an efficient traveler who has a lot of energy to see multiple things in a day. With a plan and a selected 5 or 6 main points with sightseeing inbetween, you're good to go.

I think Yellowstone, as suggested by one person above, would make a good spot to visit off the coasts. Depending on your schedule you can add Grand Teton NP. I would try to wait until after Labor Day (Sep 7 this year) since I think the crowds/pricing will be better but maybe others have a different idea. Probably this would mean East Coast, then Seattle. From Seattle it looks like there are flights through SLC to West Yellowstone, or alternately direct from Seattle to Bozeman (80 miles from Yellowstone).

That would involve driving around the Yellowstone area but would not stop you from making your "main" driving be from SF to LA (maybe with a detour after you pick up the car from SF, to go north to the redwoods before heading south).

There's also plenty to see between Seattle and SF; now that you have lots of suggestions you may want to try to plan out the days and see how much time you have left over.

Being from Philadelphia, I'd have to agree that it should be a stop between NYC and DC. The original White House and center of government was in Philadelphia before moving to DC and they city has done an excellent job over the last 15 years in making the historical areas very visitor friendly.

It is true that the East Coast is generally very hot and humid in August and early September, although we had a gorgeous end of summer in 2014 (and maybe that will happen again!).

I haven't driven from Seattle to LA but in California, a Mustang drive along the PCH (Pacific Coast Highway) is quite nice!

Good luck with your trip. If you want a guide for a day in Philly, feel free to PM me.

My two cents, in no particular order:

• DC in summer is hot and humid, as is much of the country. If you are used to hot and humid summer weather, you'll be fine. The city was built on a malarial swamp, though the swamp and mosquitoes are (mostly) gone. Everything is air conditioned in the US.
• Philly is a worthwhile stop, but having been there many times, I'd do it as a day trip from NYC. It's an hour and 20 minutes by train. If you want to take your time, you can stay overnight. You can see all the political sights in an easy day, however.
• i agree with everything people have said about the beauty of the West Coast. I'll add that, for a slice of Eastern Americana, you should take a day trip from NYC up the Hudson Valley, which is one of the prettiest parts of the country. East Coast US has a markedly different feel than West Coast US. The two excellent historical sights you might check out in that day trip are Hyde Park (home of FDR) and one of the many Vanderbilt estates.
• NYC is in a class by itself as American cities go. You can easily spend a week here and not see it all. I'd budget 3-5 days of your trip in NYC, depending on how many museums you want to see. NYC in summer is also hot and humid, with the delightful odor of rotting garbage wafting through the streets.
• Since you're not really sure what you want to do, I'd strongly suggest getting a good general USA guidebook and using that to plan. You've gotten some good input here (and will continue to do so) but it's good to get a reference lay of the land.
• If you're not going to bother with a convertible anyway, then why not just get a nice car to drive in? I'm not sure I understand the Mustang fascination.
• You could easily spend 3 weeks tooling around the West Coast as well, depending on your priorities.

I think the upper corners of the continental US are the most amazingly beautiful parts of the land. (There are some fantastic places in between as well, but if you like a lot of greenery, these are your places) I once remarked that it's like someone grabbed the country by it's edges and dipped them in magical fairy dust to make them so heavenly.

Agree on the heat. I'm born and raised in the south so I KNOW heat and NYC on a hot summer night is so amazingly gross. The heat dome and stagnant air between buildings gives no relief. It can be really thick and unbearable... so I just drink more to get my mind off it.

Thanks for the comments.
I'm thinking maybe I'll start on the west coast and finish up on the east coast thus hopefully getting (slightly) milder weather.

Unless you're talking late Sept for the east coast, it probably won't be much different.

Yeah I'd be flying back to Sydney on Sept 11.

Trying to predict the weather months in advance is a worthless exercise. Just plan your trip however feels best to you. Going to the West Coast first sounds like a good plan to me.

Are you interested in any music festivals? Food or beer fests? That's one way to start with the calendar. We have tons of amazing fests on the west coast in late summer. Also, keep in mind where you will be for Labor Day weekend as it is a huge travel time. Rates will be up and tourist attractions full. I would try to be somewhere less touristy during that time.