GPSC's guide is pretty comprehensive and can be found here: http://gpsc.duke.edu/resources/gpsc/nightlife. Here are some other suggestions:
Triangle Brewing Company gives free tours and tastings every other Saturday at 1. Check the website for the schedule http://www.trianglebrewery.com/
DukeLemurCenter - Center gives tours by appointment. http://lemur.duke.edu/tours/
EnoRiverState Park - Lots of hiking; also has the quarry for swimming.
Durham Bulls - $7 will get you a seat and sometimes fireworks.
Frankie's FunCenter- Beer and go-carts. http://www.frankiesfunpark.com/
Piedmont Biofuels, located in Pittsboro, gives tours of its industrial facility the first Friday of every month. Sign up on their website for more info. http://biofuels.coop/