When it hasn't rained for a week or so, you can go to Mercer County Park (It gets boggy very quickly). There is a trail map posted online (please note that it is not posted through the park system site) but the last time I went months ago, there were still a number of downed trees across the park.
You can also try Sourland Mountain. Both are a little more than the flat terrain common to the area.
If you are looking for a more casual ride, you can hook up with the Lawrence-Hopewell trail from the D&R canal at the Brearly Path.