Split Rock Reservoir/Wildcat Ridge Wildlife Management Area - Morris County, NJ
One of my favorite places to hike, trail run, and kayak. The reservoir itself doesn't allow motors, so the water is nice and quiet. There's a trail loop around the reservoir, which extends out into the WMA that's great for trail running/hiking (get some nice elevation change going on with the singletrack, just non-stop rolling hills). Plus, since it's a WMA, you get a lot of wildlife - I've seen beaver, fox, bear in the WMA, bald eagles on the reservoir... but maybe the coolest is the ledge known as 'Hawk Watch'. Located at the top of a ridge, you can look out at all of the different raptors just lazily floating on the upcurrents of wind, looking around for their next meal. There's even a website that links to what raptors have been viewed that day - http://www.wcrhawkwatch.com/kiosk.html.
Definitely recommended as there are a lot of small trail loops that can be formed to hike and run, as well as the multisport water aspect as well. Biking is allowed in sections, too.