One of the best things about New York state are their parks. They keep their parks in excellent condition, including the state park on the north part of Seneca Lake.
Seneca Lake State park is lovely. You can see from the pictures that they have a huge playground with bathrooms right next to it. There is also a substantial sized splash pad next to the playground. We only paid $7 to get in, which was $1 per person. On the weekends you can use the beach and swim in the lake. They only have lifeguards on the weekends.