Really good suggestions from the previous posters. A couple of other fun activities, depending on your interests:
- Mountain Biking (ideally) or Hiking (secondary) in Crosby, MN at the site of the re purposed Cuyuna Range iron mines. Really neat, almost surreal landscape with the iron oxide colored soil and crystal clear lakes. Fall is a fun time to ride & hike.
- On your way to Crosby/Cuyuna perhaps stop and do a fishing launch on Lake Mille Lacs.
- Rent bikes and ride the Gateway Trail from one of the St. Paul trailheads heading northeast. You can enjoy a light sandwich/beverage at the Gateway Trailside food truck. Alternatively, if you want a taste of Deutschland in MN stop for some German beer/food at Gasthaus Bavarian Hunter, also off of a spur of the Gateway trail. (biking back might be a little tough if you go too heavy on the cabbage
- Carpenter Nature Center in the East Metro can have some nice fall colors and hikes, with great views of the Lake St Croix.
- I would reiterate the North Shore - although it has been said many times already.
It is nice to see suggestions that others have, many of which I have yet to try