You guys are probably tired of hearing of this, but probably hiking over 4 days and 90km to Machu Picchu, and then spending 3 and a half hours making the ascent of Machu Picchu mountain. Hiking at high altitudes was not something I was ready for.. It was definitely the toughest thing I ever asked my body to do.
It was insanely crazy and by the time it was day 5 and we were making that ascent I was sort of in a weird trance. That's what it felt like anyway. My left ankle was hurting, my muscles refused to move, I was chewing on coca leaves, and.. all sorts of chemicals were washing through my brain and body (I have no idea what the brain releases in situations like these, but it was releasing a whole crapload of stuff. I could feel it.)
So I felt weird. But finally getting to that top was such an unbelievable feeling, I'd say it qualifies as the "coolest" thing I've done. The climax of the hike was me reaching the summit and erupting in joy. The last 5 days were behind me.. such crazy amounts of walking, in such crazy (for me) conditions.. I couldn't believe I had done it. I felt feelings at that moment that I have never felt before, nor I have felt since. It was an indescribable experience and I will never forget it. The views down onto the Machu Picchu ruins were amazing too.. as were the views of the surrounding mountains.
It was a magical place and time. If I had a teleporter, that's the place I'd want to return to right away - the summit of Machu Picchu mountain.