Well, sightseeing is good and all, but there are times when you want to blow off some steam and can't be bothered to go online and waste time in order to meet someone. Here are some places where you can enjoy lighthearted, direct fun with both local guys and tourists:
1. The saunas
Athens has three saunas to offer:
Alexander Sauna in Gazi, conveniently close to all of the gay clubs, in a near-unmissable location, provides most of what you'd expect from a European sauna. A small Jacuzzi, two saunas, a steam/foam labyrinth room, a dark/naked room, a bar, a few glory holes, some porn viewing areas and relatively small cabins, free or paid depending on the level of privacy you want to have. The facilities are spread across 5 floors, so you should be prepared to use the elevator or stairs quite a bit. Upon entry, you will be provided with a free towel, condoms, slippers and your locker key, while you can also ask for a second towel later on. Keep in mind that you get a reduced entrance fee if you bear proof that you're under 30. Most crowded late-ish (and operates until the morning on Friday and Saturday).
Flex Sauna in Monastiraki Area is in a more "discreet" location, about a 5-minute walk from Monastiraki square and Ag. Eirinis Square. Labelled as the "after office" sauna, it's mostly crowded early in the afternoon, and offers a steam room, two saunas, two dark rooms and a number of cabins. You will get your locker key, free towels, slippers and condoms for the entry price, which is particularly low and gets even lower on special events. The place is relatively smaller than Alexander, so you can't lose someone you may like as easily.
Attraxx is also in Gazi, right next to the new S-cape club. It offers a labyrinth with cabins scattered throughout, wherein you can choose a selection of porn for your viewing pleasure, with your partner of choice.
2. The parks
Zappeion park is in Syntagma and gets quite a bit cruisy at night. Most people gather in the areas with the trees around the central fountain. (Google Coordinates)
Pedion Areos is near Victoria station and gets cruisy mostly from escorts and people working out there (running etc). Be extra careful here if you visit late at night. (Google Coordinates)
3. The beaches
Limanakia is the most popular gay beach in Athens, located right after Vouliagmeni, and it's as close to Athens as you can get for a decent swim. You'll see a few cars parked on the right side of the street in 3-4 different places. Pick one (preferably not the first one), and go ahead. It's mostly a nudist beach, and it's rocky, so don't expect to rest in the sand. You also have to dive right in, no toe-dipping for you here. When it comes to cruising, though not exclusively gay, it's quite easy to get some action, but you'll probably be seen (whether you like it or not).
Mauro Lithari is quite further away, at the 47th km of Leoforos Poseidonos, which means it is to be avoided on weekends. It does offer a more typical beach where you can lie down, enjoy the sun and get in the sea one step at a time, however, it must be noted that the water is not deemed healthy to swim in according to this year's measurements. As for the cruising part, you can meet guys all day, but the action takes place late-ish, if at all (this is the least cruis-y out of all the spots in this guide)
That's all for now....have fun and play safe ;)