Ayia Marina, signposted and about a 30-minute walk east of Spetses town, is the best, and busiest, town beach. It has a number of tavernas, cafes, bars, including the locally famous Paradise, and discos. West of Spetses town, Ayii Anaryiri has one of the best sandy beaches anywhere in the Saronic Gulf, a perfect C-shaped cove lined with trees, and, increasingly, almost more bars and tavernas than trees. The best way to get here is by water taxi. Paradise beach is crowded, littered, and, for me at least, not very appealing. Now, I hate to mention this, as all of us travel writers try to keep some secrets, but Zageria is as undeveloped a beach as you will find here. If you are interested, ask around for directions. Whichever beach you pick, go early, as beaches here get seriously crowded by midday.
Fans of The Magus may want to have a look at the beach at Ayia Paraskevi, which is bordered by pine trees. Located here are a cantina and Villa Yasemia, residence of the Magus himself. West over some rocks is the island's official nudist beach.