St Maarten and San Juan are both extremely well set up for just walking though there is far less car traffic in St Maarten. When you do get out and walk, be sure to explore the streets back away from beach. There are great shops, street vendors and restaurants for about 5 blocks back from the beach and about 20 or so blocks across. And because all of the roads here are narrow and one way, it's very easy to walk around. There is a great water taxi service for $7 round trip that drops you off in the center of the shopping. Easy on/off, or if you prefer you can walk right out to the shopping directly.
We had nothing planned here but i would recommend the America's Cup excursion if you are looking for something unique to do. We did that a few years ago and had a ball. They are the actual 12 meter boats from the mid 80's and you do a race out in the harbor. Good fun. And you still have plenty of time to explore the island.
There are two MUST stops for us in St Maarten. The Guavaberry shop and The Yoda Man shop. We highly recommend the Guavaberry coladas though they are a little strong So be aware of that. All sorts of great flavored rums and hot sauces in that shop, it's on the right side of the beach about one block back. Can't miss it, it has the image of an Uncle Ben type of character with a top hat and a sign proclaiming the World Capital of Guavaberry. You can sample everything before you buy.
I will say we came out with a bottle of their Mango rum as it was outstanding.
The Yoda Man shop is literally what it says. It's the guy who created the Yoda character for The Empire Strikes Back for LucasFilm. When we visited four years ago the shop was in the Old Street area, but he has moved to a larger space that now includes an interesting museum with Star Wars and other action film memorabilia. Today I got the chance to actually meet Nick and we chatted for about ten minutes. Super nice guy and if you are a Star Wars film fan at all, don't miss this shop. Very cool full sized Darth Vader among the neat things in the museum along with a host of facial masks of a lot of famous folks I picked up some artwork today too that he personally signed to me.