My friend and I were bored so we messed with the greybeards. If you're bored this is rather amusing. Go to the High Hrothgar Courtyard and provoke one of the greybeards. Go to a ledge and wait for him to hit you with ice flesh. If you do it right you'll fall off the mountain and if you roll fast enough, your character will start freaking out and flying everywhere. Then when you finally land you'll lie there for a while and then when you thaw out you'll be completely fine (or not doesn't always work). I suggest standing on the ledge next to the stairs where you learn clear skies.

Or how about this. You make them unessential and behead them and just gloat at their beheaded corpses.

^

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OP, i might try that when i go up there sometime soon.

My favourite bit of fun with the Greybeards is to get the nearby Frost Troll to chase you inside High Hrothgar and watch them tear him a new one!