We had season passes for Kings Dominion in 2003. We went to Kings Island (this was the park the Brady Bunch filmed a show at) in OH on Labor Day weekend for 2 days and had a great time. Since we had the passes to Kings Dominion, admission was free, although we did have to pay for parking both days. I think $9. The parks are similar in some ways (Eiffel Tower, Paramount Theater, Scooby Doo coaster), but Kings Island has the 2 best wooden roller coasters in the world there, well, at least the longest (The Beast), and the tallest, fastest, and only wooden looping coaster (Son of Beast). It was worth the 8 hour drive just to ride Son of Beast! We've never been to Carowinds, but they don't seem to have any WOW coasters like Island does. We've been to Paramount's Canada's Wonderland too, a few years ago. It was similar to the others, but no really special rides there either. One plus is that they allow you to bring in coolers/food/drinks there. That park is right outside Toronto.
We went to Cedar Point 2 years ago on Labor Day weekend and had a blast!! If you like coasters, this is the place to go, as they have 16. They have the tallest and fastest steel ones in the world (Top Thrill Dragster--over 400 ft tall, 120 mph, Millenium Force--over 300 ft tall, 90 mph). The park is gorgeous, sits on a peninsula on Lake Erie, and you can walk outside the park and go to the beach if you wish. If you're going to drive 8 or so hours to go to a new park, I'd definitely suggest Cedar Point. If you already have season passes to Kings Dominion, I'd go to Kings Island. If you have the time you could do both, since they are both in OH. They are a few hours apart, however.
Other nice, and closer choices, are Busch Gardens or Hershey Park. Both about 3 hours or so from here. I'd avoid Six Flags. It's a pretty park, but the rides constantly break down, at least every time I've been there.
Sorry this post is so long. As you can probably tell we are coaster enthusiasts! Hope some of this was helpful