I went zip-lining with a friend for her birthday at Navitat Adventures in Writewood, CA. I loved it. The people are very professional, and I felt very safe as I jumped out over 30-story drop-offs (not that my heart wasn't racing). The guides were also very knowledgeable about the nature we were zipping through/over and shared all kinds of interesting facts. A great way to spend a day, just make sure you go to the bathroom before you put the harness on (I was pretty uncomfortable by the end of the tour).