You are going to one of my all time favorite places!
I figure you are driving and are in for a real treat if you do! First of all, take your time and watch your speed. The road can be crowded and you want to enjoy the sights along the way.
There is really a lot to do in Key West and I would recommend the following:
Jimmy Buffets, great value and unique place on Duvall street.
Sloppy Joe's and Hog's Breath Inn, across from each other on Duval street.
Mallory Square at sunset! It's wild but wonderful.
All the Marriott properties are very nice and good values. Consider using their transportation, as parking can be a problem.
Have a great time!
Even though it's been years, I agree with everything Jerry said. I went snorkeling in Key Largo and took a glass bottom boat, but spent most of the time camping (yes, me -- it was long ago and far away) near the ?7 Mile Bridge? But along with an ex I spent tons of time in Key West. It is absolutely fantastic. You have to watch the sunsets above all, but then there is Hemingway's estate, etc.
If you drive from the mainland be prepared for a few spots of dreary smack dab on the road tourist sites, crammed together in an ugly fashion, but then as you head further south you'll magnificent vistas of azure blue waters and wonderful coral reefs starting at or near Marathon Key. Patience is required if there are RVs on the road, but slow and easy is the best way to get to Key West.
As a pilot and owner of an aircraft, I fly down to Marathon often for lunch. The ride down there along Miami Beach and the upper keys is great.
I imagine a ride in a biplane with an open cockpit would be even better.
Dont stay at the Fairfield Inn Key West if at all possible. The rooms were small, poor internet, linen was filthy, and the food court area smelled horribly from the adjacent parking lots dumpsters.
Spend the xtra and get a suite from the courtyard or 400ish for the town place Marriott on the water.