There are some things I always loved to do as a kid in Ann Arbor...

• Work in the garden (or help out at a community garden)
• Walk through Fuller Park to Gallup Park past the great play structure.
  • Don't feed the geese
  • Rent one of those paddle boats
• Canoe or kayak down the Huron River (you can start at Bandemer or Argo Dam and go to Barton Dam).
• Go out of town to Hudson Mills Metropark and stop at the Dexter A&W on the way back
  • Visit the Dexter Cider Mill
• Teach knitting
• Play at Eberwhite, it has the best playground in the city.
• Pick fruit
• Hike through Black Pond Woods and visit Leslie Science Center
• Visit the Hands-on Museum
• Build a sandbox
• Go to the library
• Play Sim City: "Residents demand a dog biscuit bakery!"
• Go to Open Swim at the Indoor Pools
• Go to Ingalls Mall (not a shopping center, at the base of Burton Memorial Tower) and run around, throw pennies in the fountain, walk in front of Hill Auditorium and try not to step on red, stop at ERC and get a sprinkle doughnut from Washtenaw Dairy and a small milk for the kid, with a tasty caffeinated beverage for the adult.
• Go to the AADL downtown and do the current craft project. Choose some books and read them aloud, either in the comfy chairs in the picture book section, or out in the garden if weather permits.
• Wander around downtown and look for fairy doors.

There's someone else's ideas of what to do with kids in Detroit you can use for more inspiration.