Grand Rapids (65 km east, about 1 hour) is the obvious day trip — Michigan's second-largest city has the excellent Frederik Meijer Gardens and Sculpture Park, the Grand Rapids Art Museum, a very strong craft brewery scene, and more dining options than Muskegon. The city has reinvented itself well over the past two decades and rewards a full day.
Sleeping Bear Dunes National Lakeshore (about 120 km north, roughly 1.5 hours) is one of the most dramatic landscapes on the Great Lakes — massive sand dunes rising 100 metres directly above Lake Michigan, with long hiking trails, quiet beaches, and the Dune Climb that draws visitors from across the Midwest. Allow a full day; the dunes and the scenic drive through the national lakeshore take more time than they appear to on the map.
Saugatuck and Douglas (50 km south, about 45 minutes) are a pair of small art-colony towns at the mouth of the Kalamazoo River on Lake Michigan. Galleries, good restaurants, a strong LGBTQ+ tourism scene, and better beaches than Muskegon in some respects. A comfortable half-day trip.
Holland (55 km south, about 50 minutes) is a Dutch-heritage city with notable tulip festivals in May, a historic windmill open for tours, and clean Lake Michigan beaches at Holland State Park. More family-friendly than Saugatuck and very busy in tulip season.
Milwaukee, Wisconsin (accessible via the Lake Express ferry, 2.5 hours across the lake) is technically a day trip if you catch an early crossing — Milwaukee has a good art museum, the Harley-Davidson Museum, and a waterfront worth walking. The round-trip ferry fare is not cheap, but the crossing itself is an experience.