D&R canal - stop for ice cream or something along the way. don't make it a forced march either! maybe start in manville - go 30 minutes south, 30 minutes back, then hit a pizza place over by the airport.
going longer, bring some food, start in bound brook, stop at six mile run - maybe ride into the blue field trail. have lunch on the bench (1/2 mile in?). ride back.
Good luck! make it an adventure.
i'd also recommend the columbia trail, out of high bridge, ride to califon. stop in general store on top of hill, have ice cream, ride back. See gnomes, the ken lockwood gorge, etc...