Looking For Ideas

I am looking for cheap and fun things to do with a seven year old and a three year old. I would love things to do with the one year old also but will hire babysitters for him occassionally. Things are going fairly well with our new little sister, P. I just want to find more free or very cheap things to do in the cold Wisconsin winters. Let me know if you have any great ideas. We seem to do better with structured activities than just hanging out at my house cooking or doing craft projects.