I live near Kennywood, and agree that it is a 1 day park. Several coasters, several water rides, and all your typical amusement park rides. Plus plenty of Goodies like Potato Patch fries and Funnel cakes!
That said, if you're looking for more than one day, SandCastle is also owned by Kennywood, and is a short distance away. It's a water park, and tickets can be purchased individually, or a "hopper" type pass can be purchased for both. Please see options at the link!
kennywood.com/plan-a-visit/tickets-and-passes
Also, a slightly further distance away but also owned by Kennywood: Idlewild Park. This is in Ligonier, PA, roughly an hour away from Kennywood. But, if you have kids, this is the place! Basically, it's a giant Kiddie Land aimed at the smaller kids. And, it also has a water park.
As to discount tickets, most people in the area get their tickets from Giant Eagle, a local grocery store. You might have them in your area? Not sure how far north they go. Anyway, you need an Advantage Card to get the discount (free grocery store discount card). I'm honestly not sure how the prices compare to other company's discounts, though. More details here:
gianteagle.com/Save/Community-Savings-Rewards/Family-Fun-Savings
Sorry for the lack of Hyperlinks, but I'm still too new to post pics or links.

Hope that helped a bit, though!
