The Great Wall Of China..
If you have never been, go, it is worth a trip to China for sure. Think about the sheer amount of human labor it took to build something like this across the top of a mountain range! Staggering. Also, be sure to run at a group of aging tourists and scare them, always good for a laugh.
You can easily take a cab from Beijing or hire a driver for a day for a reasonable rate. Odds are they will take you to one of the touristy spots with lots of people selling stuff and maybe even a ski lift so you dont have to walk up to the wall and then you can take a luge down. I would highly recommend asking your driver for an out of the way spot so you can check out the wall in peace and really take in the serenity and scale of it.