We have a daughter and her family living in S Florida, so we spend a lot of time there with many trips down the Keys with the next one to a rented house in March. We once spent 10 days in Key Largo. It's a very small place with good beaches nearby and interesting eateries. The trick is to find them. KL has a very restrictive sign by-law and you literally have to hunt places down. John Pennekamp State Park is just down the road with it's coral reefs. Great for snorkeling or scuba diving. A bit further down the road is Islamorada where one of my favourite seafood restaurants is located.
Or if you're up for it, 3 hours or so will take you all the way down to Key West. Once you hit the bridges further down, the road is one of the prettiest you'll ever see with the most amazing turquoise water. How to describe Key West.... Colourful is probably the first word that comes to mind. It fits both the buildings as well as it's inhabitants. There are no good natural beaches here but there are plenty of other activities to make up for it - great snorkelling and scuba, Mallory Square at sunset, the nightlife can't be beat.
Beth