it's mostly been said i think ... but here's mine:
rock climbing - practice on an indoor wall, or as suggested, do traverses at a low level. I think it's useful to be lightly built (but not essential). Also, as suggested above, it'll be much easier and safer if you get a real climber to start you off. Find a club or something. The thing i remember most about rock climbing is having forearm muscles very tight after every session.
backpacking - again as said above, walking with a 25 Kilo backpack is probably the best practice there is ("train hard - fight easy"). But any exercises that strengthen legs, back, shoulders and to some extent abs muscles are good. And you should train for endurance, not for pretty muscles
and unless you like pain you'll probably prefer to get into it gradually, wearing you boots in etc etc.
and learning to pace yourself when walking (especially with heavy loads) is pretty important too ... i'd try to acquire that skill a.s.a.p.
have fun