I'm from the eastern side of PA and ride to the central part a few times each year, but never go west of it much.
There's a lot of good riding to the east of Erie. The sunsets from the beach on Presque Isle State Park are said to be some of the best in the world ("Second best place to see the sunset in the USA")- something to do with the either the atmosphere and being on a penninsula, or with PA people who like to brag. The short ride through the park, (which is in Erie) is really nice. Here in PA, we don't charge an addmission fee for our parks.
I just visited the Allegany National Forest, and Tionesta area last month. Rt. 666 is a nice road, and most others through the National Forest are scenic. The northern end of 62, Rt.s 27 and 36 were good too.
The northern, middle third of PA is one of the most undeveloped areas here in the North east. (look at the PA Map)
In Cooksburg, PA you can take the road that goes through the Cooks Forest State Park along the river, and keep following the road, staying allongside and north of the river till you get to Ridgeway. (It might be slow going through the park in-season, particularly on a weekend day, but early in the morning and during the week last month I had the road all to myself). From Ridgeway, Rt. 120, 255, and 555 are not to far, which are good places to see the PA Elk herd. Maybe a days ride out and back. The Benzette/Medix Run area is a good place to see the Elk, along 555. There are maps available online I think. The Medix Run hotel is popular for food and beer stops.
Anywhere you go out there, there are state parks with distinct natural feature to stop and see.
Rt. 6 is not a decent road for scenery, which takes you to Wellsboro and the PA Grand Canyon. The "Grand Canyon of the East" in New York is not too far from Erie either (Mount Morris, NY). I think either one is a days ride out and back, and maybe a loop of both would be a long days ride.
Rt.s 872, 44, 144 run north/south between 120 and 6 and all are pretty scenic.
The road through Hyner Run state park, 1014, (as well as the view itself) is pretty distinct. Takes you from Rt. 120 to Rt. 44.
Niagara Falls are pretty close but no good road to get there maybe except interstate.
A scenic, long day loop would be to take and east road 19-8-36-949 (or the river road through cooks foerest) to Ridgeway-120E-255S-555-120E (OR just stay on 120 if you didn't want to see elk) to 872N (or 44N through Hyner Run if you have time)-6west-948s-666w-62-erie
I didn't have time to edit my post as I just got called away.