Recollect all the good times you guys had (kind of like a little timeline presentation...) , set something up where each memory is behind a door... blah blah, at the end, open the last door with the ring behind it and propose away... h:

EDIT: Put some shit in there saying how each memory was like stepping into a new adventure/experience and whether the outcome was good or bad, it made you a stronger couple... blah blah...