Sitting on the beach, reading a book, etc. is what we do half the time. Go claim a palapa early - one of us gets up at 6-6:30 to hang a bag at a palapa to secure it.
Go snorkeling - the resort is sort of meh for snorkeling but it's right there. There are lots of cool places to snorkel on the island - Eden Rock is my favorite, but we try to avoid it on days when there are multiple cruise ships in. Cemetery Beach and Macabuca are other favorites (there's a dive shop next to the Cracked Conch / Macabuca Tiki Bar restaurant with a built in set of stairs into the water for divers/snorkelers) It's a great dive spot but the snorkeling there is cool too. There's a shelf right there that drops off 300-400 feet or so for the divers but the water is gorgeous for snorkeling too - go snorkel, then pop out for a few drinks at the bar, then go back in.