I went to Santa Maria, which is 30 mins outside Havana. Its one of the cleanest around, and I was quite impressed. You can get there by taxi (you can negotiate with the cabby to drop and pick you up), or they have a direct bus from Parque Central leaving every 20 mins and costs 5 CUC – round trip. The bus was pretty convenient.
Although I didn’t visit it, but you can also go to Guanabo beach. There’s a guy who will take you on a dinghy to go snorkelling – they claim the reef he took my friends to was one of the most beautiful.
Alternatively, you can go to Club Habana in Miramar. About 45 mins away, it’s an exclusive club with a private beach, pool and clubhouse. Expats can get a day pass for about 40 CUC a head. Worth it if you have some company and want to spend a tranquil day at the beach, otherwise Santa Maria should do to. You’ll pass all the embassies in Miramar on the way.