Apostles are very nice. I was there last year (
here's some pics a friend took of that trip) although the weather didn't cooperate well. Haven't been to Isle Royale but I hear it's pretty awesome as well. On the way to the Apostles, we stopped off at Pictured Rocks, and wished we'd planned for more time there.
This year I'm planning to head to Georgian Bay in Lake Huron. It's supposed to be pretty cool too.