Go quite frequently. First, you have to go to Sunset Cigars. Great place to pick up some sticks, shoot the breeze, etc.
Restaurants - well, that's entirely what you are after.
A very nice place is Michaels. Definitely make reservations there.
Also have some Key West staples like Louie's Backyard and Alice's. Both great.
Martha's is ok...kind of a been there forever type of place, that's not too bad.
There is also this fish hut, literally a ramshackle old wreck kind of place, that I always, always, always forget the name of that's down by the piers. OUTSTANDING fish sandwiches there. Great place, inside is really just covered patio area...you place your order at the counter, then listen for your name to be called and then go pick it up. Inside has license plates, signed crap all over the place, etc. Maybe Emo can remember the name of this place better than I, but I always know how to get there and I always go everytime I'm there.
Have fun. Key West has definitely changed since you've probably been. Not necessarily for the better IMHO. Just way different from its glory days. But, you'll get out of it what you make it, so enjoy brother!