Ahh......to be young again. I would love to hike to half dome in Yosemite National Park but I don't think I'm in good enough shape to do it. It's a 17 mile hike with some really steep areas where you have to hold on to a cable(click here). I suppose if I got in better shape I could do it but it will probably never happen.
There's an interesting blog here from someone who took the hike.