South Mountain Reservation in Millburn, New Jersey. It was, naturally, a hike that was pretty rather than one that offered major drama - it's not the Grand Canyon - but it was consistently pretty, with quite a few surprises along the way.
The 5.2-mile loop hike is easily accessible via NJ Transit, and it's a good one for novice hikers. The hardest part occurs straight out of the gate, when you have to head uphill on the Lenape Trail. Survive that, and the first major reward is an overlook of Millburn. The overlook is probably much better in winter, but there are two interesting markers at the spot: One for where George Washington supposedly stood, and a rather enigmatic memorial to a friend of the park.
The trail continues pleasantly around the top of the "mountain" to Hemlock Falls, pictured above. This is the midpoint of the trip and a nice spot for lunch. You can take a set of stairs above the falls, although there is nothing spectacular up there.
The return follows the Rahway "River" (more of a creek, really). The trails widens and eases up, but don't think the return trip will just be a trudge. The river widens into a pond and just after that is an abandoned pumphouse.
Buildings like this are usually sensibly locked up so no one hurts themselves and sues. Not this time! I was squeeing with glee to discover it was open and I could wander around, trying not to fall through the rotting floor, to my heart's content.
After that is an old bridge shored up enough for pedestrians, then a dam. Not long after, you end up where you started.
On the weekends, trains run from Penn Station once an hour, so you have to time it carefully (and note that fare is $8 each way). The vehicularly endowed can choose to drive, of course. Either way, this hike is a genuine hike that takes full advantage of the land it is sited on.