I really liked the one at BWoods when I did it with a friend who was deathly afraid of heights. The first zip is about 3 feet off the ground, and start to finish about 40 feet long. They are teaching the group members how to use the harness, and how to land on the platform at the end. The next one proceeds from that landing platform, and is a little longer and a little higher. Each zip is progressively longer and higher off the ground.
The design eases the group into ever so slightly more height and length and speed, slowly, so as to not freak anyone out. It was perfect for my friend. The thing we were learning was how to manage our speed so we could land on the next platform without missing it entirely (going too slow gets you sliding backwards when you miss the platform, so you end up hanging motionless stuck out in the middle of the line) or slamming into the platform out of control. Rotating on our line while moving forward was always a possibility on these zips, so not rotating was something we learned to do too. As they take you through all the zips, the progression includes three rappels and two rope bridges, so you're not always just doing the same thing over and over. Since you're in a group, you get to wait on the rest and watch their reactions to each activity, and cheer on the people who are feeling cautious. It felt more like an educational experience than a scary carnival ride.
The last true zip was the long one with the valley way below. The big challenge was to get to the end without rolling backwards (which we really didn't want to do and get stuck waaay out there), and get onto the platform without coming in too fast and crashing. Another challenge was to turn our heads left and right to enjoy the view without prompting our whole body to rotate, while zipping along at our highest speed of the day. We congratulated ourselves at the end for a job well done and skills learned.
The last feature was a "zip" without any skill needed - just let go and see how fast gravity takes you down, paired with another person on a second line - racing. You can't rotate on this one; they take care of that for you in the way you're hooked onto the line. Our leaders explained that how you hold your body can help reduce wind drag, but that how much you weigh counts more than your aerodynamic body position, So sorry Galileo, there's also wind resistance to consider. The heavier person usually wins. At the end of that zip, we were charging very fast into a building and I thought we'd die crashing. The mechanism stopped up magically. There was a crowd watching and waiting and clapping and cheering.
This whole adventure was perfect for my fearful friend. She was as stiff as a log the whole time, but proclaimed strongly that she had fun. I don't think she'd do it again, though.