If you're doing Kennywood, Darien Lake and Cedar Point, I'd recommend a few hours at Waldameer.
It wouldn't be to far off your route it seems and you don't have to pay to park or enter the park. If you choose to do everything at Waldameer and hit it on a calm day you could probably be done in a few hours. I know I've done the whole water park in about 35 minutes on a slow day.
You can choose to pay for what you want to ride and if you need a break from the car Presque Isle is right there and has some great Lake Erie beaches.
Darien Lake is typically a half day park if you are only doing the dry side, it's been years since I've done the water park, just be careful to try to avoid a concert day unless you want extended waits, they only have one train for all of the roller coasters (except Tantrum).