After visiting Georgioupolis, we thought we'd investigate this lake in close vicinity, but hadn't really known what to expect (assumed just "a lake"). How surprised we were to find it was like a mini beach, with canoes and pedaloes, as well as sunloungers, parasols and the usual array of tavernas and souvenir shops. Possibly a little too commercialised, but it wasn't overly busy the day we were there. There were people swimming in the lake as well. We did take a pedalo out for an hour (didn't think €12 was too bad for an hour between the 4 of us), and were pleased to have some good views of goldfish, turtles and a heron. The weather on the day we went was slightly cloudy, but it made a refreshing change from the heat of the beaches on other days.
You can easily spend a few hours here if you include a pedalo trip, lunch, swimming and people watching!
As others have said, don't be duped into thinking that the 1st car park is the only free one, there are plenty more closer to the lake if you don't want the walk.