With respect, I don't really think you can say Sentosa has nice beaches. The sand is course and the water dirty.
The beaches at Bintan are above average. There is a really nice beach at Pulau Sibu, Malaysia which is only 2-1/2 hours from Singapore (2 hours by car/rest by boat).
If you want a treat take a long weekend in Koh Samui, Thailand. The beaches along Chaweng are the amazing, as good as Bali, with crystal clear water. Great island to relax, direct flights from Singapore, cheap accommodation. What more can you ask for?