I've done a lot of the climbs within my capabilities on Tryfan in the past and I've stood on the cannon. If you're into this sort of thing you could also visit the Cantilever Stone on Glyder Fach - not too far from Tryfan. Other notable places to stand on and have your photo taken are The Chioch and Inn Pin (Skye, see the film Highlander), top of the Bridestone (near Todmorden). There must be loads around the country. Others will be able to suggest more, I'm sure. My favourite personally, is a picture some kind soul sent me of me, my wife and dog, silhouetted against the sunset on Blackstone Edge. The dog was one then, now 17. If I think on in the next few days I'll get my ancient scanner going and post it on this forum.