Possibly Kennywood? That should be a must!
Best time to go, if you don't already have your dates planned, is mid-week in late June or July. The largest crowds are at the beginning of summer when school is out (there are many school picnic days, so check their calendar to see when they are to avoid them) and at the end right before school is back in. Being so close to Pittsburgh, the park is widely visited by the surrounding school districts.
When you're in the park, arrive early and head to the back of the park first, and work your way to the entrance. Many people don't go for the rides in the far back first (which happen to be the best). Snag a ride on Exterminator and Black Widow first, those lines build quickly. The Phantom's Revenge doesn't usually exceed an hour unless it's a busy day. The 3 water rides are less busy early on and later in the day, but close after dark. And check if there's a light show the night you're going, because the Jack Rabbit and Racer close early on those days. Make sure you get a ride on Skyrocket before you go, it may not seem like much but that little ride is the smoothest coaster I've ever been on!
MOST IMPORTANTLY:
DO NOT FORGET THE POTATO PATCH FRIES! And of course, don't forget to brush up on your
Pittsburghese or no one will know what you're talking about
sandwich = "sammich"
did you eat yet? = "Jeet-Jet?"
soda = "pop"
gyro= "jy-ro"
you all = "yinz"
etc. etc.
Hope this helps! I've been going to Kennywood 2-3 times a year since I was about 5 so I can help you out if you have any other questions