Mark Check out the link:
North American Hostels If you are traveling solo, I would recommend hitting some hostels as you will probably meet a good mix of Americans and Europeans who are also in travel mode. I took 10 days two years ago, after doing the Alcatraz Triathlon and cruised around Northern California. Stayed in San Fran, Yosemite, Santa Cruz. The hostels in Yosemite is very good and the hiking is awesome. I recommend this - but could get lonely on your own.
There are a lot of states that have their own Outdoor Activity Websites which list many places to go bungee jumping and sky diving, etc. etc.
If you flew from Chicago to Las Vegas and rented a car there you could drive to the Grand Canyon in about 4 hrs. This is one of the Wonders of the World.
I've heard that Eugene, Oregon is party place. If you are going to Seattle, then you may want to look into this.
If you can fly around San Diego -> bottom southwest tip of America is AWESOME weather, beaches and 'kini's.

Plus you can go to Mexico right across the border for some partying. (Just don't go too crazy.)
North of San Diego is L.A. which is a HUGE city geographically. It does have some good beach towns, just make sure you ask around and find one that is easy to hang out in.
Tempe, Arizona - right outside Phoenix - and not too far from L.A. is a good party town. This is also a good spot to hit if you go south from the Grand Canyon.
I ~believe~ that there are place to go skiing out west all year round, too.
If you have time to drive I would check head east across the country from one of the above spots and you can hit Yellowstone up North or New Mexico in the South. New Mexico has many Pueblo Villages featuring some of the oldest Indian Tribes in America.
New Mexico also has some of the Largest Caverns in the world. If you are down south in New Mexico check out the Sand Dunes in Alamogordo
check out this site:
Sand Dunes in the SouthWest Ausin, Texas is a great town and known for its bar-music scene.
New Orleans is a fun city and a VERY interesting city as the Spanish, French, Creoles & Indians all left their mark there. (Will be VERY hot though.)
Bring lots of good CDs as the roads are EXTREMELY LONG. The western states are huge.
Of course
you should try to make it to the Eastern seaboard where there are great beaches all up the Alantic. Virginia Beach, Fire Island, NYC, Florida. The Florida Keys have some serious partying going on, as does Miami.
I would recommend making a few different itineries as you may want to change it on the fly due to weather or in case you meet up with some folks traveling in different direction.
Hope it helps...