As a coaster enthusiast and having extensively travelled the US to ride coasters, here are a few MUST visit parks...
Cedar Point (Sandusky, OH) - probably the best coaster collection in the world. (2 days)
Kings Island (Mason, OH) - my favorite larger park and with the new for 2020 Orion will rival Cedar Point. Also home of The Beast...nothing like it and my favorite coaster to ride at night. (1 day)
Holiday World (Santa Claus, IN) - the first “theme” park (1946) in the US...each area of the park is themed after a major holiday...Halloween, Thanksgiving, Christmas, Fourth of July. They also have one of the top two water parks in the US. Finally, the reason you HAVE to go...the Voyage...the best wooden coaster on the planet and my favorite (204 coaster credits and counting ) coaster period. Their other coasters are excellent and very well maintained. The overall park experience is truly amazing...my wife’s favorite park. (2 days if you are doing the waterpark)
As far as the west coast (California) goes, I just spent 10 days there last summer to visit Six Flags Magic Mountain, Six Flags Discovery Kingdom, California’s Great America, and Knott’s Berry Farm. The ONLY one I would visit again is Knott’s Berry Farm. Magic Mountain was disappointing and considering it’s reputation is very overrated, doesn’t hold a candle to Cedar Point which it is often compared to. The other two were duds.
Hope this helps...good luck with your trip...you are sure to have a great time here!