Swim at Tham Phang Beach , the busiest beach of the island. It is quiet on week days and a lot more populated on weekends and holidays. Half of the beach is sandy and after it becomes more rocky. There is a restaurant, you can rent a kayak and do banana boating
Look at the sunset at Laem Chakrapong, interesting also during the day for the landscape. It is alos a popular fishing spot. You can also admire the sunset at Chong Khao Khad
Snorkeling at Koh Khang Khao (Bat Island) , a small island in the South end of Koh Si Chang with an attractive beach. See with Pan & David Restaurant if you are interested in snorkeling there.
Visit the other islands Koh Karm Yai and Koh Karm Noi, 800 meters from Koh Sichang. About forty families live there. The ferry to or from Si Racha can bring you there but you have to tell the staff that you want to go to Koh Karm.
Explore the island , you can rent a Tuk-tuk for the day to explore the island. It will cost between around 300 baht.