Michigan's Adventure, about 10 km north of downtown Muskegon on US-31, is the largest amusement park in Michigan and includes a substantial waterpark. It operates from May to September and is primarily aimed at families and thrill-seekers — a mix of coasters, flat rides, and water slides. It is owned by Cedar Fair (the same company that runs Cedar Point in Ohio) and the roller coasters are a step above the average regional park.
The attached WildWater Adventure waterpark is particularly good in summer — wave pool, lazy river, and multiple slides. A combined ticket covers both parks; arrive early in summer as parking and crowds build quickly. Not a reason to come to Muskegon specifically, but a good option if you are in the area with children.