The surface is cinders/dirt - not great on skinny road tires, but fine on moutain/hybrid tires. Parking can be a problem on warm spring weekends early in the season, and during "leaf season" in the fall, when the trail gets heavily used. Generally, the further north you go, the easier it is to find parking, especially north of Lambertville, NJ. Washington Crossing, NJ is a nice place to start, with some parking. Plenty of restaurants, deli's, taverns, etc. in every town along the tow path if you don't want to bring victuals - Frenchtown, Stockton, Lambertville, Washington's Crossing all have restaurants and taverns.
Just across the river is PA's Delaware Canal - running from Easton, PA to Bristol, through the famously-artsy town of New Hope. That surface is more dirt than anything else. Accessible via several bridges across the Delware from NJ to PA.
PLEASE CHECK CONDITIONS BEFORE YOU GO! Both paths suffered damage in the recent flooding. Here's the report from PA:
9/22/2004 12:10:00 PMDUE TO EXTENSIVE FLOOD DAMAGE, THE MAJORITY OF DELAWARE CANAL STATE PARK IS CLOSED. The only area that is still open to the public is from Route 532 in Washington Crossing south to Bristol. Other than that section, the park is closed. The flood has done extensive damage to the towpath and canal structure and the park staff is still assessing the full scope of the damage.
For NJ: dandrcanal.comdandrcanal.comOpen ↗
For PA: http://www.dcnr.state.pa.us/stateparks/parks/delawarecanal.aspx