If the weather is nice I recommend renting bikes and taking then around the Three Rivers Heritage Trail. You can either rent one of the Pogoh bikes or from somewhere like Golden Triangle Bike Rentals. On the trail I would definitely go to point state park, around the north shore and through the strip district. Those call all be done in an hour or two on bikes if you don't stop.

Otherwise Oakland is great, with the cathedral of learning and the campus feel. Lawrenceville is also a great spot to walk the 20 or so blocks along Bulter St and stop at the overwhelming amount of bars, restaurants and shops