Okay, I did it! Or rather, my Sims did it. And it wasn't all that hard - they got it on the first try. (Well, actually, the third try, because the first time, I didn't realise that when I scheduled the event through the phone, it was going to start right now. And, the second time, my Sims were waiting for it to start at Onsen, and it got cancelled because they were supposed to already be up at the top of the gondola when it started.) If you schedule from the calendar, you can pick the date and exact time the event will start.
I chose Spring for the big climb, as I figured it would be the best bet for weather. They did run into one thundersnow storm - on the ginormous wall, of course - but they were so elated when they completed it, they totally forgot to 'run inside'. (Which would have been a bit difficult on a mountain climb.) Carl warned about holidays, however: make sure that you don't have any within the two or so days you start, or your event will be suddenly cancelled. I think it took my Sims less than a day to get to the top, if I figure six hours for each stage, and they started at noon, but they stayed a couple of nights at the top. Probably, I should have scheduled the event for 6 am, as they finished the last wall in darkness.
Anyway, I'd created these two Sims for the sole purpose of climbing the mountain (if you have a husband and husband, or any two adult Sims in your household, you don't have to invite any nasty townies along). I'd got them both 10s in mountain climbing and fitness, and they also had the 'seldom weary' trait. For some reason, my 'energetic' sporty Sim is the one who constantly fell off the walls and got himself injured, so I had his husband, the 'genius', do the 'champion climb' (or whatever it's called - you only have to have one Sim do it, and the other can 'climb cautiously').
The 'tasks' aren't silly or superfluous. Mainly, you have to reach the next trailmarker, get a little sleep, eat something, and climb some walls. I had trouble pitching a tent in the second phase for their nap (which they need after getting through the first part because it sucks out half their energy), but finally discovered that there is already a tent they can use at that level - it's just before the bridge and second set of walls. Also, 'gather Sims at the trailmarker' is the first task listed, but it's actually the last thing you do, because the trailmarker is after the walls. 'Pause' is your friend, if you don't want your Sims wandering about aimlessly looking for the next wall.
I did have trouble finding the cave at the top, and I never tried to find the one that is supposedly at the second level. If you are looking at the summit from the trail below, it's on your left; rotate your camera clockwise and you'll see a little triangular group of rocks that changes your cursor.
I also built a little cabin at the top of the summit (you can go into build mode even though you have Sims in the area). This is rather useful, as there are no trash cans at the top, and my Sims were piling their rotten leftover food near the pee bush. (You would think that an eight-person dish of grilled chicken would remain fresh forever at sub-freezing temperatures, but no.)
