At Loantake Brook Reservation in Morris County, the bike route is scenic and basically flat. You’ll pass a water crossing, a playground and duck pond, and beautiful woods. It’s a loop trail, so you can ride the entire distance (10 plus miles) or turn around at any point for a shorter trip.
You might also enjoy riding along the level road at Island Beach State Park. The 16 mile roundtrip is beautiful in any season, but if you go in the spring or fall, you won’t have to shar the trail with as many cars. But a word of warning: it can be a bit of a workout on a windy day.