The Delaware Seashore State Park is by far the BEST state park in Delaware. If you're looking to hit the beach and cool off in summertime, it's the best possible choice you could make. Here's the plan: head out early as there is limited parking (and thus limited visitors) with a car full of family and friends and, of course, a well-stocked cooler. Yes, there are some snack bars in the park if you're looking for a cold drink or some chips, but your best bet is to bring the beach umbrella, blanket, and food yourself and have a lovely picnic in the sun by the coast.
If you want to make a weekend out of it, there are also plenty of cabins for rent in the park and RV hookups. If you want to rent a cabin, just stop by Indian River Marina and inquire or, better yet, call ahead to make sure there is room available. I'd suggest the cabins: they're fully-furnished with TV and AC and you can get a real Atlantic seaboard experience. If all the relaxing in the sun and eating has you a bit restless, there's a nature trail that loops around the wetlands at Burton Island Nature Reserve that leaves from the marina. It's a good place to clear your head, take in the fresh air, and see the birds. Another trail to consider is the Thompson's Island trail. Thompson's Island is full of massive oak trees that look like something out of a movie and was actually a sacred place for some of Delaware's Native American tribes. It probably offers the best bird-watching in all of Delaware Seashore State Park.
So, whether you're looking to kick back on the beach with a beer and a picnic, kayak the Atlantic ocean, or just enjoy the great outdoors, Delaware Seashore State Park is your go-to place in Delaware.