A few years back we rented bikes and rode from the cruise port out past Maho. There is a hell of a hill in between that we ended up walking our bikes up and the roads were a little scary but the islanders are used to bike traffic and make allowances and I never felt threatened, intimidated yes, but not in danger.
We did stop at Maho beach just to get the thrill of the planes coming in almost on top of your head and it did make for some great movies/photos but our time there was limited because we did not enjoy the crowd, and it is always crowded.
We ended up riding down to several other beaches parking our bikes, getting wet and the occasional drink. Later that afternoon we dropped our bikes off at the shop in Maho and took the public transport back toward the port, that is an adventure in itself article on public transport. We got off way before the ship at the far end of the boardwalk and walked back along the beach enjoying the shops, scenery and the local brews along the way. It was a pleasant stay overall but I will say that most of the beaches you are going to see may be crowded depending upon how many ships are in port that day and many of the shops are the typical shops you see in every cruise port, set up to scalp cruise passengers. The days of quaint local village type shops are long gone unfortunately.
Might I suggest Trip Advisor or Cruise Critic (check out the forum here) for other suggestions.