I always recommend Old Fort Niagara (American side - short ride to Lake Ontario). The fort is very historical (dates back to French and Indian War times). The "castle" is the oldest building on the great lakes. The fort sits within a state park with a beautiful sloping lawn down to Lake Ontario. You'll have beautiful views of sailboats and on a real clear day you'll see the Toronto skyline.

There's no swimmable beach, but there is a rock/sand/driftwood filled shoreline. It's a great place to look for beach glass and old pottery and rocks. There are also swimming pools.

All in all it's a great place to spend an afternoon.