Perhaps a Tree House would be fun. It doesn't have to be high up and we can always have stairs if necessary (instead of a ladder) Here are some inspirations:
Tiny House Blog's 3-33-11 post
freshome.com's Top 8 Most Amazing Treehouses - unrealistic for us, but feeds our creative minds!
Realistically the township would probably not like us with this one. This is an interesting option and while I wouldn't rule it out, I also would not be quick to rule it in. Plus, my mom doesn't do well with stairs and I think there would be some folks who would not fair well with any form of heights. This is really fun to consider though, and perhaps someday we could build a sweet kids tree house if we have enough mature trees on our land. Perhaps it could second as a guest house. ;-)