Go see the Fairgrounds
From Thursday to Saturday you will find the Farmer's Market at the Fairgrounds of Allentown packed. From the butcher stands to the produce to the kiffles (an Amish pastry cookie-type snack) the Farmers Market has the freshest food you can find. Aside from the market shopping, though, is the great people you see as you walk around. Locals will come by, grab a seat at one of the lunch counters and gab for hours.
The best part of the market are the baked goods prepared and sold by the Mennonite families who live nearby. I strongly suggest the kiffles and the Goods potato chips.
The market is only open on weekends: Thurs 9am-8pm, Fri 8am-8pm, Sat 8am-6pm. No parking worries on any day, the lot is quite large.
Next to the market area (in the same building) is a small flea market and some dollar store-style shops. Next to the market building is the Roxy, a great spot for ice cream and burgers.
During August the Fairgrounds host The Great Allentown Fair. Complete with rickety rides, games and wonderfully bad for you fair food. It costs about $5 for adult admission but if you are attending one of the concerts that go on during Fair week you can get into the Fair for free.
Fair concerts are usually quite good. When I was young I saw Paula Abdul and in recent years the Fair has hosted My Chemical Romance, James Taylor and Gretchen Wilson. Tickets for the concerts are usually low (compared to the big venues in Philadelphia where most shows go) and since the concert arena is large tickets are often still available at the last minute.