The homepage of the Delaware and Raritan State Park has a link to a printable map which also shows parking locations along the towpath. This link has other information; camping areas, park activities, interesting places to visit along the canal, and the canal history and an address to get a map and guide to the park.
We bicycled on the canal between the southern end near Trenton and Blackwells Mills near the northern end. The canal has a smooth surface most of the way except for some sections south of Princeton where there is coarser gravel. We rode with mountain bikes having smooth road tires. Hybrid bikes would be okay. When riding bike paths, we like to ride one way on the bike path and then follow back roads on the return trips. We can usually plan a good route even in unfamiliar territory using the Delorme Street Atlas software. On this trip we followed the path in both directions after finding heavy traffic even on some back roads. A good loop route takes a little more planning here.
Here are a series of pictures and descriptions of a bike ride along the Main Canal Towpath.