I’ll put in another vote for Pictured Rocks. I have great memories of a solo camping trip to Grand Island, too. Oh Hell, let’s just say the UP in general. I get lots of funny looks when I tell all the people in Colorado (where I currently live) that the hiking/skiing/etc. is better in the UP than it is in Colorado, but for some of us, it is.
Colorado is gorgeous - you get beautiful vistas, lovely mountains, etc. etc. But there’s one thing that always spoils it for me - there’s too many damn people out here! Unless you drive to the boonies, then hike in past the reasonable day-hike line, you’re hiking in a crowd. Plus, there’s all the rules/regulations/etc. that have to be enforced when a spot is so popular. Give me the UP any day, where you can hike naked and never worry about running into anyone else.