Tioman offers all of the typical beach activities, such as swimming, snorkeling, etc. The snorkeling is particularly good, as almost every beach has its own shallow reef, and just about every resort will have masks and fins for rent.
Anemones and their cute little friends can be found all around Tioman
As good as the snorkeling is, the diving is even better. While it won't quite match the very best sites such as Thailand's Similans, the diving at Tioman is extremely enjoyable, and there are more than a dozen great sites within 30 minutes or so of most resorts.
Perhaps the most unexpected activity you'll find on tiny Tioman island is golfing. The Berjaya Tioman resort includes an 18 hole golf course. It appears that the course is open to non-guests, but for convenience you may wish to stay at one of the Berjaya resorts in any case.