I live on Marco Island. It's not that good for late night doings since the streets pretty much roll up around 9PM (after sunset and you can't be on the beach after sunset because of turtle season in the warmer months). but restaurants are good, there are some good bars, pizza, Italian food places. Many of the restaurants offer free cab rides to their place from the beach side places (a few timeshares, three hotels, lots of condos). Shelling is good on Marco, especially if you are up really early, like sunrise. And the sunsets are magnificent on Marco.
December is high season, so Marco is hoppin' . You almost need a reservation to get to the Publix store. The restaurants are open longer and have almost no days they are closed like in Summer. The Gulf can be cooler, so you might not be swimming as much and you can only use pools at the beach side places before sunset. Everglades (Shark Valley) is about a hour way.