You won't be docking- cruise ships anchor off VG either near Spring Bay area or North Sound. Do you know where you will be tendering ashore?
With that port time you should be able to do quite a bit!
There is a beach at BEYC but water entry is pretty rocky and there aren't many beach chairs. But food is good and they do a fun happy hour. Watersports are available too. I would choose BEYC over Saba any day but you have enough time to do both.
If you want to do something truly different have you looked into the Nature Tour over to Necker? You have the time.