On Saturday, September 29th it is National Museum Day and a variety of museums across the country are offering free admission that day.
You can get all the details here. Make sure you print out the museum card, otherwise you won't get free admission that day.
As a note, there are none in WI - which is where we'll be then. But for my friends still here in MN, you can go to the Minneapolis Institute of Arts, or Stearn Cty History Museum (St. Cloud).
Oh - I guess after looking at the MIA website, they are always free anyways.
For my known readers in NY or CA or IL, there are A LOT to choose from. Have fun at the museum that day.