Dig the possibility of a built in ball or water toy. Maybe something we can add ourselves. Thinking that a peg-board or similar on the wall (outside, inside, both) would give us some flexibility here. I just realized this is a cat play house, but the one that inspired me earlier was for having balls roll down much like at wonderlab.
DIY - Simon DeMott kitty playhouse - Adapted from Martha Stewart's cardboard cat house template. http://www.marthastewart.com/921522/how-make-cardboard-cat-playhouse
Ken Wingard has a fun DIY that kids will love. DIY Hobbit Hole Playhouse for Kids. Customized playhouses can cost up to $4,000 but using Ken’s instructions, this playhouse will cost you only $200. It also makes a great weekend project that you can work on with your kids!