Using this day as an "onboard spa day"

We are cruising Celebrity Summit's southern Caribbean very intensive port itinerary. By the time we arrive in St Maarten, we will have had several full day tours under our belts (and hopefully some late night fun!), so we may prefer staying onboard enjoying some down time. A couples massage using our onboard credit sounds perfect.

Let me just say that we've been to St. Maarten before, a full week of visiting many attractions on our own with a rental car. We enjoyed many beaches, scenic spots, local fare, planes, water sports on both Dutch and French sides - bot nothing around the cruise terminal area.

That being said... we may consider coming onshore for an hour or two if there is something close/walkable that is a must see, especially being the ship is in port till 6pm, the latest of all ports. Any ideas for a quick fun spot to enjoy? Thx!

Unless you are into shopping, or want to spend some time on a so-so beach, Phillipsburg has very little to offer. While the island itself strikes me as having more than any other, the fact is, if you are "islanded out" , there is nothing wrong with a port day on board. I just would have chosen another island for it.

We can fit our spa day into the sea day, but thinking the ship will be crowded and not as relaxing on deck after service, or else do it on St. Thomas (was going to do Magen's Bay, paddle boarding). I appreciate the comments, will probably wait to book it until we get onboard anyway, see if they have deals. Thanks.