Visiting Machu Picchu shouldn’t be a problem if you are afraid of heights if you take the train to get there.  However if you hike the Inca Trail to get there be aware that there are some steeper areas and some exposed areas on the trail.  This means there are chances of loose rocks and unsure footing along the trail so take each step cautiously. However there are no real drop-offs and the trail is several feet wide and set well into the mountain sides. Some of the stair sections are pretty steep, and if the trail is wet there is a chance it might be slippery.