Koh Larn is a popular day trip island just out of Pattaya. You can go there with a regular passenger boat on your own, or you book a speedboat trip. We did the later, organized from main hangout point. Around 20 people joined for a day on the beach, and in the water with food and drinks. A fun day.
The beaches can be overcrowded at certain times, when the boats come in every 30 seconds, but later the day beaches can literally be deserted. Good luck. You can also do a lot of water sport activities like the banana boat, jet ski or para sailing.