Watkins Glen and Letchworth State Park are both fabulous. Letchworth is much larger; more than one full day would not in any way be too much there. Watkins Glen can be comfortably "seen" in a day, at least the major highlights.
Knoebels amusement park in Elysburg is a fun and (relatively) affordable stop, too. They have two campgrounds directly associated with it: Lake Glory is a couple miles away from the amusement park, with shuttle service a few times a day. The Knoebels campground is directly adjacent to the amusement park, such that you walk back and forth. The Lake Glory campground is overall the nicer campground, with bigger sites and a more campground-like atmosphere, but I prefer the convenience of walking to and from my site and being able to (for instance) take a quick nap in the afternoon.
If you stretch your timetable, you could see all these in a nice loop (Philly to Knoebels to Letchworth to Niagara Falls to Watkins Glen to Philly, or the same in reverse), but for a week trip it's probably too much to do comfortably.