We really enjoyed camping at Rock Creek State Park this weekend. I highly recommend site 151 for tent camping. It's the last site along the lake, which makes it more private and quiet.
The park has 8 miles of trails. They're mostly wide grassy trails that wind around the lake, but also some narrow dirt trails through wooded areas.
The swimming beach is located on the opposite side of the lake from the campground. It has several parking spots nearby, picnic tables and a vault toilet. The water was great during our visit, but I recommend checking the beach water quality before swimming.
Rock Creek Marina offers boat rentals, but only has one double kayak available, so we didn't rent kayaks during this trip. It was a handy place to purchase firewood though. It's $12 for a wheelbarrow full. The also have a mini store and sell food.