There are plenty of interesting shops and sites, including: good coffee at Small World Coffee. grab a PHAT lady hoagie at Hoagie Haven, ice cream/cup cakes at Bent Spoon or TSweets, sandwiches/cookies at Olives, and good burgers/beer at Triumph Brewing Co.
Check out all the great work at the Princeton Art museum
Get your fall on at Terhune Orchards.
Princeton's a pretty gorgeous campus to wander around if we do say so ourselves, including some great architecture, and if you make it far enough, a lake, boathouse, and amazing rugby pitch.
Grab a historical walking tour of Princeton.
Get on the water for a bit (probably not the best lake to swim in though.)
A nice trail along the canal for a run.