We just spent 5 days in the Keys at the end of February. My husband and I loved it! It's quite different than mainland Florida. We rented a convertible, and loved driving the overseas highway. So scenic and beautiful.
We flew to Ft Lauderdale and drove from there. Stopped at a
Walmart on the way to pick up a styrofoam cooler, drinks, snacks, etc. We spent a couple nights in Key Largo, and visited the Everglades (highly recommended). Wanted to snorkel in Key Largo but it was quite windy and our tour got canceled. Then spent a night in Key West, went on a sunset cruise (again, highly recommended). I wish we'd spent another night in Key West, but the hotels there are quite a bit more expensive. We splurged for that one night. Then we stayed in Marathon for our last night. Went to Bahia Honda state park, and took the snorkel trip to Looe Key. Very nice. Spent some time on Big Pine Key wandering around the Key Deer preserve. (they are small deer only found in the keys). Managed to see 3. Some are apparently friendly and will come right up to you, but not the ones we saw.
Key West had a lot more going on than anywhere else in the Keys. But if you wanted to save on the hotel cost you could stay on a nearby key and just drive in. There are lots of things to see and do in the keys, you'll probably want to explore a bit. You won't regret the driving, seeing the ocean a few meters away on either side of you is great!
The beaches are not the white sandy beaches you see on the mainland. There's sand, just lots of rocks when you are going into the water. Of course the trade off is the snorkeling. The water gorgeous, so blue and clear. There are tons of charters for fishing, snorkeling, diving, catamarans, etc. There is also a day trip to the Dry Tortugas that you can take from Key West. We would have gone if we'd had more time.
Be prepared, restaurants are not cheap. Lots of fresh, delicious seafood, but it was a bit more expensive than we were expecting. Hotels are more expensive during the winter months, but I did find some good deals on hotwire. Also got a great deal on our car rental using expedia. Just had to prepay for the rental, but it was less than half price. Also, if you drive, do not speed! The state troopers are literally everywhere on the highway. Very heavily patrolled. You can fly directly into Key West on a few airlines, otherwise you'll have to fly into Miami, Ft Lauderdale, or Ft Myers and drive. We found Ft Lauderdale to be the cheapest.
Hope this helped a bit!
Christina