Lums Pond State Park is one of the most popular state parks in Delaware and a popular place for fishing, kayaking, or just having a picnic in the fresh air. Fishing is probably the most popular activity at Lums Pond; the lake is quite large (200-acres) and while there are plenty of wild fish like pike and crappie, the park service makes sure to keep the lake stocked with freshwater bass.
The park is also large enough to offer some good trails and even cross-country skiing and snowshoeing in winter. The best trail is the Swamp Forest Trail which surrounds the entire pond and takes you by all the major places in the park (boat launch, Frisbee golf course, etc.) so it's not a bad way to discover what the park is all about.
Finally, I'd like to mention the recently-inaugurated Go Ape Treetop Adventure Course. It's a ropes and adventure course up in the trees and open to the public (this is, if you're not afraid of heights!). Don't forget that there are nearly 70 campsites at Lums Pond State Park, most of which are set up for tent camping, and even a couple of yurts if you're looking for something different.