Cherry Creek Park is one of my favorites. It's close to town, yet miles away in "feel". On an ambitious day, I will ride my bike from Denver down the Cherry Creek Bike Path to circle the reservoir. The path is well-maintained and easy to navigate.
Once inside the well-kept park (it seems I see staff cleaning and maintaining every time I am there) there are numerous places to picnic, hike, camp, fish, boat and swim. The bathrooms are numerous and CLEAN! The swim beach is located just northwest of the east entrance and it is a popular summer spot for dry Colorado. Note that the park often has to limit the number of visitors so it's best to come early on hot summer days. There is a playground, concession stand and first aid station.
Even if you are not a boater, it's fun to watch the boats on the lake. There is also a model airplane filed and a Prairie Nature Trail, all of which make for a full day!