We didn't do any excursions on the ship and found our own adventuring just fine.
We walked into the city and around the castle, El Morro, and watched the most brilliant sunset. There wasn't a lot of time since the boat arrived a few hours late (5pm) and we only had until 10pm. We adventured our for some Mofungo and found it at El Jibarito...we laugh because I think we got Mofungoed and had quite a belly ache for the rest of the night. Still, it was a lot of fun.
For St. Maarten/St. Martin we grabbed a taxi to Orient beach and had a great day. It was a 7 dollar cab ride per person for a 20-30 minute drive up to the French side for Orient beach. They drop you off right on the beach, go right for nudity or left for a more conservative experience. We walked up about midway, rented a jet ski to cruise the ocean for an hour (if you do this watch the time, we paid $90 for 70 minutes and only got 45 minutes because we weren't wearing a watch!) The chairs are reasonably priced and the drinks are as well.
We also grabbed a taxi to Marigold and had lunch in the Marina...also a great time. I talked with many people who did excursions (submarine adventure, etc) and were very disappointed. I'd highly recommend creating your own adventure! Too much waiting around for the excursions!
We jetted out to Megan's bay and had a great time relaxing. Easy to get out there on taxi and very easy to get back. What a beautiful spot, just make sure you keep walking, and walking and walking to the quiet end of the beach. They you're sure to have a great time!