A major factor in Porto Galinhas' popularity is the collection of amazing tidal pools full of tropical fish that appear in the coral reefs around 100m in front of the town during low tide. It's possible to wade out to the pools but the local government limits numbers on the reef at any time, so you'll need to get a free wristband at the Praça do Relógio and wait for the next group. Take a snorkel to scrutinize the abundant marine life.
You can rent snorkels with masks on the beach for 1½ hours (R$10, with fins R$15).
If you don't want to wait or wade through the water, it's also possible to get out to the pools on a small sail boat. Boat tours tend to visit part of the reef just to the north of where the DIY visitors congregate.