Isle Royale National Park in March

The park is only accessible from mid-April through October, but realistically, June through September are your best bets. June brings wildflowers and fewer bugs than you'd expect, but nights can still hit 40 degrees. July and August are warmest – highs around 70, lows in the 50s – but that's also peak mosquito season. September is the sweet spot: bugs are dying off, fall colors start showing, and you'll have more solitude. October can be beautiful but unpredictable; storms roll in fast and boat service becomes irregular. The Ranger III stops running after early October, so check schedules carefully if you're planning a late-season trip.