My wife and I did a zip lining in Monteverde during our honeymoon stay in CR. My wife is afraid of heights and was not looking forward to this. This turned out to be one of many highlights for her on our honeymoon. The company picks you up at the hotel and takes you to their office to get you fitted in the gear. I recommend wearing comfortable shoes, shorts and shirt and light jacket. You are in the trees most of the time, so the sun is blocked. There is also a little hiking to do. The staff was super friendly and helpful. They made sure that everyone was having a good time and knew what they were doing and that they were comfortable with it. There was a total of 6 people in our group with 2 guides. One of the guides took each couples camera and took pictures and videos of everyone the whole time. You go on I believe 12 zip lines, repel down about a 100 plus feet and then climb inside a tree to get back up to the platform. They also have tarzan swing to start the adventure out to get your comfortable.
It was honestly a blast and having such a professional staff made it that much more fun and safe feeling. The views you get on some of the lines are just unreal. A couple people saw some white throat monkeys, but they ran away by the time the rest of us got to the platform.