Just until I can't wipe myself any more - whether that's 60 or 80 the way I look at it, when I can't poop independently then its time to go. I'm thinking I'll go skydiving, enjoy the fall and the beautiful scenery and then aim for a big, industrial strength wood chipper which will be turned on to "Sequoia Sawdust" mode. Simple, painless, good for the earth and no clean up other than rinsing out the wood chipper.
Personally I would just go the skydiving route and skip the parachute. The free falling would be exhilarating and like you said the view incredible.