I leave the country for vacation on May 29th, so I'll still be in Missouri when you get here and there's a pretty gosh darn good chance of me making it to Carlisle this year.
Gas prices will be 25-30% higher in the summer, so prepare for that, and motels and hotels will also be slightly more expensive during that time. There's a few one-way car rental choices out there, and Budget seems to be the most flexible. Budget typically lets you drive unlimited miles to any state without restrictions. The one-way rental costs will vary since their goal is to basically get you to move their cars to markets where they need them.
You're more than welcome to borrow my cars and go wherever you want. I guess the dilemma is how to get it back.
I'm sure they do this in Australia as well -- the rental company will try to scare you into buying additional insurance coverage. They'll even lie and try to say the car has no insurance unless you buy their special coverage, which isn't true at all [or legal]. I would skip it. Bring a copy of your car insurance papers with you in case they give you a hard time. I've only had that problem with Enterprise; Budget has always been easy for me to deal with.
Between California and Pennsylvania you could fit these into your route:
Old Route 66 -- various historic structures and small museums in Arizona, Oklahoma, and New Mexico. Lots of stuff you'll recognize from the movie "Cars". Old 66 runs mostly parallel to interstate 40.
Grand Canyon
The St Louis Arch -- its not very good but the elevator ride in the little rotating egg is cool.
Music in Nashville. You'll want to look up mr1der and meet up with him.
If you're into US presidents, you could visit the museums/libraries of Ronald Reagan and Richard Nixon (California) or Bill Clinton (Arkansas).
Wal-Mart has a small museum in Bentonville Arkansas if you're into folksy rags to riches stories.
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whole bunch of stuff in Washington DC
If you're into car factories, greasy food, and music, I strongly suggesting fitting Chicago and Detroit into your trip. You can go back and tell Cathy you survived a night in Detroit.

Chicago is an essential food stop, especially Portillo's and Giordano's.
Detroit is home to the Motown Records Museum and the Ford Rogue factory where they build trucks. For $7 you can tour the factory at your own pace and watch them build vehicles. There's also a Chrysler museum in Auburn Hills MI which I think is free. GM's renaissance center is downtown. There isn't much there but its a great place to take pictures and you can look at a couple cars displayed in the lobby. Lafayette's is nearby where you can order a "coney dog", a hot dog drenched in chili. Despite the poverty and mismanagement, its actually a great city.
Both Chicago and Detroit are on the way to Cleveland, and Cleveland is home to Jerry, Stephanie, and all those NSR guys as well as the rock and roll hall of fame. Apparently Cleveland's food claim to fame is a monster pastrami sandwich which I have yet to try.
Our friend Wes is in central Indiana.
The scenery gets pretty boring once you get to my neck of the woods (its all corn fields) but gets interesting again once you get up to Lake Michigan or eastward to Pennsylvania. Something you MUST experience if you drive in the US is the Pacific Coast Highway along the California coast. Texas is pretty flat too, but in a spacious and big sort of way.
If you see billboards for silly roadside attractions, then by all means check it out. Usually those are the silly little gems that give you a taste of the local culture. The interstates are safe but boring. During the day, its worth seeing if there's a state road or scenic route that runs parallel to your path that you can enjoy for an hour or so. At night, stick to the interstates.
Lodging -- There's probably no better bang for the buck than Motel 6. They're clean, friendly, and most of them have free internet. In the middle, south and southwest regions of the country you can stay in them for as cheap as $35. The quality of the particular location will depend entirely on how nice of a neighborhood its in.
Food -- Start by eating at In and Out in Los Angeles. As far as I'm concerned, its the best fast food burger in America. Chicago, Detroit, Cleveland, Washington DC, and Philadelphia are significant places for food.
Rest areas in the west are sometimes terrible. In Nevada they're basically plastic toilets mounted on top of a hole in the ground (with a plastic tank underneath full of chemicals). In Idaho they sometimes don't have toilet seats or doors on the stalls. In the east they're bright, clean, and secure. Out west, I tend to prefer taking a dump at a truck stop or McDonald's.
There will be toll booths in Oklahoma and Pennsylvania, expect to spend $18 in tolls to cross Pennsylvania.
Motel 6 used to publish free travel guides with listings of all their properties with nightly rates and maps. Not sure if they still do, but there's always motel6.com. Rest areas are also a great place to get brochures for local attractions and diversions as well as coupons for hotels.
I use Priceline to get higher-end hotels at low rates but on a road trip it requires a little bit of planning and an internet connection. Their cutoff for same-day reservations is 9 or 10pm so you have to think ahead about what city you plan on being in when you're ready to sleep and book ahead accordingly.
If you go to Las Vegas, you should hit up my brother on Facebook for tips and ideas on free entertainment. He just got back a couple days ago. You can see talk show tapings for free and food is cheap.
To save money, there's KOA campgrounds all over the country with showers and security. Camping is definitely not my thing, especially with summer heat. You could park at a rest area and just sleep in your car if the area seems safe and secure. I've done that before in a pinch but again, summer heat is a concern. There's usually lots of activity at rest areas from truckers parking overnight so they don't feel so spooky.
Wal-Mart stores allow RVs and campers to stay overnight in their parking lots. Just go in and ask the manager if its okay if you're unsure. They like the business generated by travelers. Again, summer heat would keep me from doing that.
Give me a call if you're lost and wandering the continent!