Haad Thong Reng is a quiet and laid-back beach in the east of Koh Phangan away from the crowds. It's a bit of an adventure to get there but it's worth it!
Here's what to do:
Navigate to the beach of Haad Than Sadet (also really pretty) and park your scooter there. Both of these beaches are actually part of a national park so you'll need to pay 100 baht to enter.
To the right of Haad Than Sadet there is a rickety wooden bridge that I wasn't sure would hold (but luckily did) that leads to Haad Thong Reng. After the bridge turn RIGHT. This will take you around to some stairs, over a hill and then on the other side, next to the ruins of an abandoned hotel, you'll find a secluded beach with soft sand, grass to lay your towel on (!), and beautiful, clear water. It's the perfect spot to unwind and soak up the sun without any distractions. There was also a couple selling fresh coconuts by the beach when we visited!
Unfortunately we visited on a cloudy day and during high-tide. If you can catch the beach on a sunny day and during low tide you'll have the absolute best conditions!