About 45 minutes' drive south of Pattaya is Haad Nang Ram Beach. The beach is open to the public although it's located on Royal Thai Navy land. Unlike Pattaya, or even Jomtien, there's very little development here. There is a parking lot with some food stalls, but not much else. There are a couple of small islands offshore that you can swim to, and lots of coral along the floor in parts, so this is a very good area for snorkeling.
Perhaps the best thing about this beach, and why I mention it here, is that it can be almost deserted on weekdays. Crowds of Thais can descend on the beach on the weekends or long holidays, but if you visit on a regular work day, you can have the place to yourself. You will of course need your own transportation to get there and back.