Many people enjoy taking a tour with Bernard's Tours in Sint Maarten/St Martin. There's also Joyce Prince tours.
Our first time we were there we just spent the day in downtown Philipsburg, sitting on the beach for a little bit, doing some shopping and wandering, hitting the "Yoda Guy" place (a guy who worked on Yoda and now sells art and has a little museum of sorts), and having late lunch at Hard Rock Cafe.
Second time DS and I did Bernard's and are some of the very very few who didn't enjoy it. (didn't like our tourguide and the tour was longer than what we booked and I'm still not sure how that happened)
Next time we're there we'll likely get a taxi and go to a beach. Many people like Maho Beach, where you can stand basically at the end of a runway and it's very impressive. DS and I didn't care for it (and standing at the
Avis rental car place at LAX gave basically the same experience for just the cost of a rental car LOL). Orient Beach is a popular one; note that all beaches there are very likely to be topless, and full nudity isn't forbidden. DS was fabulously embarrassed to see a lady about my age just sitting there with no top on...it was very funny.
There are several areas in San Juan that you might want to stay in. If you are focusing on Old San Juan and don't need a beach closeby, just stay in Old San Juan! I really want to stay at Hotel el Convento, but the cost when we're going is just too high for the number of days we're staying. When we're there we'll likely stay at the boring Embassy Suites and we'll just take cabs to where we want to go.
Cruises that start and/or end in San Juan generally go out of or in to the Panamerica Pier. This is different than the pier right in Old San Juan where day-stop cruises are. It's worth knowing ahead of time.
