I love going to the Keys! There is much more to do than just partying. There's many historical places to visit (Hemingway House, Lighthouse, etc.) plus snorkeling, fishing, and scuba diving, art museums, shopping... lots to do!
If you're looking for somewhere to just relax, try the Bonefish Resort (I think it's on Marathon.) I stayed there once with my aunt and it was SO beautiful and relaxing.
Like everyone else said, there's no real beaches unless you stay at one of the higher-end resorts with the private beaches... but personally, I don't think you really get the "feel" of Key West there. Old Town's B&B's are a fun way to experience it all. Most are close to the shops and museums so you can just walk instead of trying to drive (traffic is bad often) and then paying to park ($$$!)
Here are a few websites you can check out to get some more info:
http://www.keysnews.com
www.keywest.com
Keys News is the online version of The Citizen, the daily newspaper. I read it to keep up on the stuff that's going on. Keywest.com has tons of great info as well as links to hotels, B&B's, and activites.
Good luck and have a great time!