I discovered the Assunpink area thanks to NJDOT’s High Point to Cape May route and have done many variations of a loop through it since then.
This one, at close to 30 miles, starts and ends by the West Windsor Senior Center (more parking than the nearby library and about a mile from the Princeton Junction train station). It takes you past the camping stop in Mercer County Park (more here), Union Transportation Trail (read about a ride here and a nearby farm brewery) and some horse farms, through the Assunpink Wildlife Management Area and the New Deal town of Roosevelt and into Hightstown and the posh Peddie School, right past the Old Hights Brewery (bike ride here) and back into West Windsor.
You could always detour to a massive marble Hindu temple (more here), a chocolate factory (here) or Working Dog Winery.
The roads are generally quiet, you cross U.S. 130 at a traffic light and your biggest climbs might be the two overpasses over the New Jersey Turnpike.
I can’t stress enough how much great cycling central New Jersey offers … and so much variety too. Here are five great day rides on trails and a weekend’s worth of biking and more around Princeton. You want hills? The Sourland Mountains from Hopewell to the Delaware River.