If you don't want to spend a long-time riding on the boat, Koh Tan and Koh Mudsum is a good choice on your day trip. You will get picked up by minivan from your hotels to the pier and across to the islands by speed boat that it takes about 15-20 minutes. The trip is provided 2 times a day for morning and afternoon.
First, you will do snorkeling around Koh Tan, then heading to have lunch at the restaurant on the beach at Pig Island (Koh Mudsum), here you can also enjoy kayaking, swimming, relaxing, or playing with piggies. Special part of afternoon trip would be watching the sunset during dining on the beach.