If you're both mostly boulderers, and like bouldering, why not make the celebration "bouldering with a mission"? Trip to Torridon, Ireland or some Scottish island. Or how about going to Font and doing loads of circuits. A Half-Dome weekend (long weekend?) on the various easy circuits around Trois Pignons would be a good challenge, the restaurants are better there too and you can bring back a really good bottle to keep for the 50th.
For routes a "La Demande" weekend would be memorable too, but perhaps a bit too far from the original suggestion and parameters...
