Lots of great trailheads to hike or stroll. This is the closest to Mahwah, but there are many, many other great parks and reserves in the area to explore.
A beautiful lake to enjoy a stroll or lunch in one of the restaurants overlooking the water.
And of course, saving the best for last, you can explore the NJ Botanical Gardens. Our wedding venue is located inside the gardens, but there are 96 acres to explore. I would suggest parking near the Skyland Association Visitor Center and starting in the gardens just behind that building.