Yeah, it's interesting how interstate regional connections are often stronger than intrastate ones. As a kid growing up in Erie, I had been to Pittsburgh, Harrisburg, and State College (aside from small places in western PA), and then to Philadelphia later as a teen. But nowhere else in terms of the larger places in PA.
However, I had been to Buffalo, Rochester, Syracuse, Utica, Ithaca, Binghamton, Albany, Schenectady, Olean, Elmira, and NYC and other smaller places throughout NY. All before I had ever been to PA cities like Reading, Williamsport, Lehigh Valley, Scranton-WB, Johnstown, Altoona, Lancaster-York (which I still have never visited, but need to!).
Summer is the best time to go! July and August, especially (the waters off Erie are really warm usually by mid-July... low 80s). I would definitely recommend a baseball game -- it's a nice minor league park right downtown, so there's lots of bars/breweries/restaturants within close proximity. And Presque Isle is pretty much the best thing about Erie, so it's a must do when there. There's definitely a lot to do in the Erie-Chautauqua region in the summer.
The city of Erie is pretty rough. Things are looking up with a lot of downtown and bayfront investment happening... but it's rough and rusty as hell in much of the core. If you ever make a trip, PM me and I'm happy to give you some native recs!
Johnstown has some cool history, a few things to do, some great industrial-era architedcture, and it's in close proximity to some nice natural areas in the Laurel Highlands. I don't know that it's a place that I would go too far out of my way to visit... but like most of us on here, I'm fascinated by urban settlements regardless of their current conditions. The downtown is pretty depressing and dead (at least it was 2 years ago when I was last there). I would say Johnstown is more of a pop-in, afternoon visit type place... check out the Flood Museum, ride the incline, have a bite and beer, and get out of town.