For a small fee (15,000đ) you can join a 45-minute boat tour out to Hon Phu Tu and the nearby Hang Tien Grotto, which has some attractive stalactites and stalagmites. Nguyen Anh (later to become Gia Long) hid here while on the run after the Tay Son Rebellion, and locals have dubbed its stone plateaux as his throne, sofa, bed and so on. If there’s no one else around, you can rent the entire boat for about 300,000đ for this short trip.
For a more luxurious boat trip around local islands, the Hon Trem Resort can organize a full day-trip, including fishing and lunch as well as a visit to Nghe Island and the Ba Lua Archipelago, for about $150 per person. It may also be worth speaking to Hung at Tan Phat restaurant (t 016 6735 8168) as they have boats for rent and are sometimes amenable to negotiating the price, so it should work out cheaper than a day out with the Hon Trem Resort.