Bintan Island itself is more ideal for a resort vacation. Most people that visits Bintan island will be accommodating at one of the resorts on the island as there will be several recreational activities within the various resorts itself. Hence, there are not much to do on the island for a day trip and things are exorbitantly priced at Bintan and are comparable to Singapore.
If you are looking for a day trip to one of the Riau Island in Indonesia, perhaps you could consider Batam Island instead. It is just a 45 minutes ferry ride from Harbourfront Ferry Terminal in Singapore and you can expect significantly cheaper food and spa there.