Rockville Lake Park in Rockville Indiana is an excellent place to cool off on a hot summer day. With a sand beach, calm water, and a wading area which gradually slopes into a deep swimming area, it is perfect for young children (with parental supervision of course) and adults alike.
In the swimming area there is a concrete dock which you can climb up and jump off of. (Careful, it is easy to skin yourself on the concrete!) The water is deep enough for a dive!
There is a playground and picnic area near the beach. Note that pets are allowed in the park, but not on the beach.