We just love St. Maarten. If you are not looking for anything too exotic, I would suggest taking the water taxi across to Great Bay ($5.00 per person for all day, as many trips as you want) and you can browse the shops and sightsee a bit and them relax on the beach. The locals are very friendly and duty free prices are very low, possibly the lowest in the Caribbean along with St Thomas. You can rent to nice beach chairs and and umbrella for $5.00 and the waiters at the nearby bars will serve you right in your beach chair, and cheap too.....usually the beer of the day is $1.00.
If you want to go further afield, then Orient Beach is beautiful (if you don't mind nude bathers) and lots of great shopping and restaurants nearby as well.
Friends of ours took a taxi to the beach right near the airport, where the planes fly right over your head, but this would not be for the faint of heart, although they loved it.