We were in those ports last April (07)...
St.Maarten we took a taxi to Orient Bay Beach (it's CLOTHING OPTIONAL so don't be alarmed to see totally nude folks prancing around) Here's a link: http://www.st-maarten-info.com/St-Maarten-Beaches/OrientBay.html
We LOVED it...simply b/c I could go topless and not worry about a client seeing me, lol. It's not for everyone, but we had a nice time. The vendors there can be annoying but if you say no once or twice...they kinda pass the message. Other than that, we had a lovely beach day there.
St. Thomas we did a Catamaran tour (here is a link: http://www.shoretrips.com/common/search4.asp?rcode=CAR&lcode=STT&tcode=001244) and had an awesome time. It included lunch, FREE drinks, and snacks, and the snorkling was AWESOME. This company offers all KINDS of tours and we were more than happy with their service and prices. Here is the link to the other excursions (http://www.shoretrips.com/common/search3.asp?rcode=CAR&lcode=STT)
San Juan we ventured out on our own and went shopping and eating, but here is a link to the excursions offered through shore trips (http://www.shoretrips.com/common/search3.asp?rcode=CAR&lcode=SJU)
Happy Sailing!