All the advice posted so far relates to Centre County, which is where you'll have access to Penns, Spring Creek (including Fisherman's Paradise), Little Juniata, and Big Fishing Creek. Beware: Pennsylvania has about a million Fishing Creeks and nearly as many Spring Creeks, so make sure you have solid directions to the places you want to go.
I'll offer that if you want to fish Letort Spring Run, Big Spring, Yellow Breeches Creek, Falling Springs and other streams that form the core of the Limestone Legends book, a good jumping off point is Pine Grove Furnace State Park. It's on a reasonably good wild trout stream, Mountain Creek (another ubiquitous stream name), and within a few minutes of Big Spring, Breeches and Letort, and a little longer to Falling Springs.
Strictly speaking, I'd divide it into two separate trips: South Central and Center County. Letort, et al., are easily two hours from Centre County. If you're going in May or early June, hit Centre County. Any later and the big creeks (notably Penns) get a bit warm. Late June or later, I'd go with South Central.
You'll note most of the guys are focused on Centre in their comments, and for good reason: the fishing, and the outdoors experience in that territory are both better. South Central is solid fishing, but super crowded, and there's a lot of stockers (except in the Letort). It's well worth doing, though, because of the area's relationship to the history of fly fishing (probably second only to the Catskills).
Letort is almost exclusively wild brown trout and it flows through a fairly urban - suburban watershed. A great stream, but do not expect seclusion or a great outdoors experience.
Falling Springs is similar, though a bit more rural. It has wild rainbows and wild browns, and the open water in the lower reaches is heavily stocked.
Big Spring has wild brook, brown and rainbow (though not necessarily in the same sections), and flows through a largely rural - agricultural territory in the upper section, and a more small town urban setting in the lower (brown trout) area.
Yellow Breeches has a lot of wild browns, but you'll need to avoid the fly section if that's what you're after. It is among the most heavily stocked streams in the state, so no matter where you go, you're probably going to hit a mix of stocked and wild fish.
They're both good choices for extended trips. Regardless, enjoy yourself!
