We just returned from a week in the Florida Keys and it was wonderful. This area is not for those seeking a tremendous beach, but it has much to offer even in December-January.
We rented a wonderful home in Key Largo (the Key closest to Miami) with some other family members through a vacation rental website. It was three stories with a private pool, a home theater, and more than enough room for the 11 of us.
Key Largo is very well situated for a quick trip to Miami or the Everglades National Park, as well as exploring the other keys to the south.
First, I recommend stopping at the visitors bureau in Key Largo, because the staff there has great recommendations and many books with coupons for almost every restaurant and activitiy that you might want to enjoy.
We had fun at the Alligator Farm (about an hour away just outside Florida City, which is north of Key Largo). Admission (around $20 per adult but visitors bureau has a coupon) includes a short but thrilling airboat ride, shows involving snakes and alligators, and exhibits of different everglades animals. It was a great opportunity to get up close and personal with both alligators and crocodiles. I looked into other places that did airboat rides and many of those were two or three times more money. I thought that perhaps this shorter airboat ride might be too short, but after experiencing it, we felt it was just long enough.
Nearby is the Everglades National Park, which is a true gem. Within 5-6 miles of the park's entrance is a great visitors center with clean bathrooms and two very flat, wide and well-paved paths through different habitats found only in the Everglades. Anyone of any age or disability could enjoy these paths and see dozens of different animals and unusual vegetation.
In Florida City (between Key Largo and the Everglades National Park), you'll find some of the more common tourist items, like a massive gokart and game room place and a Prime Outlet Mall.
Key Largo has John Pennekamp Coral Reef State Park, which has trails, a small beach made up of rocks (not sand) and snorkeling/glass bottom boat/diving concessions that go out to the reefs that are 3-5 miles offshore.
I recommend finding a snorkeling/diving/glass bottom boat company outside the Park because you see the same reef areas but avoid having to pay the Park fee (which is per person, not per car) in addition to the trip fee. We used Reef Roamer, which had a beautiful 50 foot catamaran named Quicksilver. Our captain, Terry, was very honest, and told us that this particular day would not be great for snorkeling because of high winds (20-30 knots). He gave us the opportunity to back out (very unusual), and when we insisted on going, he offered to take us to an island nearby that didn't have the best coral but was much calmer. That was a very good choice. Those who went from the Pennekamp Park concessionaires that day had no warning of the high winds. My sister was on the glass bottom boat tour -- over half the occupants threw up from the rough seas and the trip was half its normal time but no discount or refund was given! She saw divers and snorkelers who spent 5-10 minutes in the water and then had to leave. We had a beautiful sail, and saw some less exciting coral, but also saw sea horses, sponges, lobsters, etc.
Another place that you might like, especially if you have children, is Theater of the Seas (within 30 minutes to the south of Key Largo). It is expensive ($20+ for adults) but the visitors bureau has a discount coupon. There are sea lion, dolphin, turtle, parrot, etc. shows and exhibits, a nice snack bar, and other activities that kids would especially enjoy.
There is an amazing bird sanctuary about 15 minutes south of Key Largo that has feeding time at 3:30 p.m. each day. Hundred and hundreds of birds come flying into the sanctuary at that time and are fed by the sanctuary volunteers. Small children might find this experience overwhelming, but my two teenagers liked it very much.
As mentioned, there are no great beaches. We checked out almost all of them within a 50 mile radius. You'll hear about Bahia State Park and how it was voted one of America's best beaches in 1992, but its beach was later almost completely destroyed by a hurricane. Some of the resorts in the area have small beaches with real sand (must have been brought in?).
Key Largo does have a nice community swimming pool called Jacobs Aquatic Center. It has a separate lap pool for serious swimmers, an area 3-5 feet deep for others, and a fun area about 18 inches deep with a pirate ship for the little ones.
