I've been here for the last four months. There are quite a lot of things to do, I guess it depends a little on what interests you.
Pinel Island is a great place to spend a day on the beach and they have several restaurants there (Yellow Beach is our favorite). You can also snorkel there. The designated snorkel area is not very good, but Little Key, which is swimmably close is quite nice and the back side of the island is also good if the waves are not too strong.
There is a Butterfly Farm which is a nice place to spend an hour or so if you like being surrounded by butterflies.
Loterie Farm on Pic Paradis (the tallest peak on the island) has hiking, ziplining and some great views. The "extreme" zipline was actually quite fun, I think the regular one might not be a bit disappointing.
Fort Louis is an old fort on the hill above Marigot and is fun to see. They recently moved the museum to the old prison nearby, so going to both will cover the island history in a couple hours.
There are lots of good restaurants, especially if you like French food.
I enjoy hiking around the island, either on actual trails or just off into the hills.
From Grand Case Beach Club, if you snorkel out past Petite Plage down towards Bell Point there is some okay snorkeling as well.
You can ride horses on the beach and out into the water, but when they get in the water, they all start pooping.
Thursday night is salsa night at Calmos Cafe. If you make it to La Noche afterward, it's usually bad news for the next day.
I found one place where some mangroves grow all the way out to the edge of a bay, so you can actually snorkel them when the tide is coming in without worrying too much about what might be in the water.
We have a lot of stuff on our blog:
Les Fruits de Mer - Extreme Shallow Snorkeling Team Everything from Nov 2009 to now is about St. Martin. If you have any other questions, just get in touch.