[winger]

I would suggest 2 days for Ohiopyle - there are lots of streams along trails (assorted difficulty). Linn Run State Park, Forbes State Forest, and another near Forbes (I can't remember the name right now, but it's right next to it) are all full of streams with some waterfalls and plenty of other pretty spots.
Weather is barely predictable 2 or 3 days out, so all bets are off trying to guess what it'll be like in a few weeks. Today was sunny and in the 70s while tomorrow is supposed to be rainy and in the 50s. Weather forecasts are not frequently correct here, I've noticed (and I'm comparing to New England). Trees have started turning slightly green in some places, so it'll be looking fairly decent in a few weeks (hopefully). Things will start a little later in the highlands (like Ohiopyle), so I'm not sure how far along things are there. I do know there are trillium along the trail up behind Cucumber Falls in Ohiopyle and sooner in a park called Cedar Creek near route 51 in the Smithton area.
Hope this helps!

edit - Laurel Hill State Park. Here's a link for the state system - http://www.dcnr.state.pa.us/wheretogo/index.htm