If you're going to be in Central PA, Penn's Creek and Spring Creek are exceptional fisheries. The Fisherman's Paradise section of Spring Creek is nice. A little more east is the Letort, which is a famous stream and accordingly gets pretty large crowds, especially on the weekends. There's also Spruce Creek, another famous stream, and although it's mostly privately owned, there is the George Harvey stretch that's about a mile long. Penn State University owns it and it is open to public fishing. If you're after large trout, Spruce is the place to go.

I'm assuming you're talking about fishing in May. Some patterns I'd recommend for the above streams are: #16 & #18 Sulphur Parachute, #14-16-18-20 Parachute Adams, #20 & #22 Blue Winged Olive, your favorite midge patterns, #12 & #14 Light Cahill, and if you are lucky #8 Green Drake. My fishing partner and I hit the Green Drake hatch last year and they looked like bats coming off the surface film. In addition to the above dry flies, Hare's Ear Nymphs and Pheasant Tail nymphs in assorted sizes and WD-40's in #20 & #22 work well for sub-surface flies.

Hope this helps some,

-Darryl