X2 on Fort Jefferson in the Dry Tortugas. I'd take a sea plane instead of boating, but that's my preference. Be sure to do a sunset cruise from Mallory Square. Don't miss the Hemingway house and the little white house.
If you like to drink, you'll be in heaven in Key West. Lot's of bars.
Keep in mind that there are very few beaches in Key West. The only decent one I know of is Fort Zachary Taylor at the very end.
I've stayed at the Sugar Loaf Key KOA several times and will be there on January 14th for a trip with friends. Then I'm off to Blue Water. Personally, I like the KOA. Yes, it is tight, but they have a nice swimming pool and I like to fish off the nearby bridge.