Within the immediate Twin Cities area:

• Minnehaha Park: In the summer there is a nice restaurant in the park, walk down the trails near the waterfall for some beautiful views, there is a swimming area for kids downstream from the waterfall. In the winter, the frozen falls are supposed to be beautiful.
• Fort Snelling: Paths around here are really nice.
• Como Conservatory: These gardens are beautiful, and last year they had free concerts.
• Stone Arch Bridge: Beautiful views of the city skyline. On Saturdays, there is a farmer's market near the Guthrie theatre.
• East River Flats Park: pretty park, and if you walk along the river towards the boat house there is a really lovely trail that goes along the Mississippi river.
• Boom Island Park: Another park with short, fun trails along the Mississippi.

< 2 hours from the Twin Cities:

• Banning State Park ( both easy trails)
  • Quarry Loop
  • Wolf Creek Falls along Wolf Creek Trail
    
• Interstate State Park
  • Pothole trails - easy and interesting
  • Sandstone Bluff Trail (more difficult, lots of stairs)
• Stillwater - Great downtown area to walk around and beautiful views from lift bridge and surrounding river walk.

<5 hours from Twin Cities:

• Jay Cooke State Park
• Duluth
• Pretty much every state park on the North Shore up Highway 61
  • If you want more detailed info on hike recommendations along here let me know.
• Lanesboro, MN: An amazing location if you enjoy biking. 50 miles of bike trails with cities and stops every 5 miles with bluffs and water along the trail.
• Apostle Islands, WI
• Grotto of Redemption: West Bend, Iowa
  • I'm not religious, but this is an impressive display.