I have the best luck with early spring and late summer/fall. Kids are still in school and parks a pretty much dead on just about any weekday. Just forget about seeing shows, most aren't operating at the beginning and end of the season. Examples: Early September at Geauga Lake last year, multiple rerides on Villian, often with nobody on the train but my group. Downside - several coasters not operating - Big Dipper, Double Loop, Raging Wolf Bobs. Cedar Point, May 7. All major rides operating, all major rides walk-on to a 5-7 minute wait. Downsides - no shows and Gemini wasn't racing.
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everything's better with a banjo