What should I put on my backyard fort?

I have a large yard for a California home, but it's not as large as the yards you East Coast-ers (and middle state people) have. My brother and I are going to build an airsoft/water gun fort, and maybe make another fort and put obstacles in between. Originally I wanted to construct a zip line, but my brother surprisingly said that would be too dangerous. So, what should we put? Any ideas for the obstacle course? We don't want to *buy* actual obstacle course items.