Ok, let's do this thing:
Climbing the Mountain
Ok, the mountain. It's big, it's windy, it's snowy, there are bad things living there that want to eat your face. You guys need to get to the top, probably because there's some ancient city or something up there. I dunno. Make something up. For reference, let's say this is about a level 5 challenge. I want to make it a big, but not hugem one, so I'm going to create a budget based off about 3 monsters. That's 192 situation points to play with
The baseline challenge here is probably going to be the ongoing storm. I could probably do something complicated revolving around decreasing temperatures or the like, but honestly, it's a to simpler to treat this as straight damage. So, let's start from this baseline:
SP: 192 (82 after budgeting)
AC: - Fortitude: 20, Reflex: 19, Will: 17
Icy Winds (Standard, At Will)
+9 vs fortitude; 1d8+2 cold damage against all targets
With nothing more than that, the challenge would be simple: Every round the storm attacks, and every round the players make skill attacks, following this general logic:
Circumvent - Take risks to do cool things like cross ice bridges and otherwise look awesome, mostly with athletics or acrobatics.
Manipulate - Really, this is just a matter of sucking it up and struggling on.
Understand - Find paths and routes that minimize exposure to the storm, using Nature or perception.
Smash - Smashing isn't really an option - you can't fight the storm, so it's not really on the table.
Powers - Using a power with a strong movement component counts as a skill use. If it has a damage component, then use that, but if not, then for an encounter power do 1d10+4 base, and for a daily, 2d8+4.
Base damage will be (1d6+4/1d10+4/2d8+4) + Stat + 1/2 level. Since I want to reward certain options and diminish the effectiveness of others. I want the rangers and such to get their chance to shine here, so use of Nature will get a damage bump of one step (Possibly up to the 4th level, 3d6+4). On the other hand, just slogging along is kind of dull, so I think it will drop damage by one step.
That's enough to cover the basics, so now we need to jazz it up.
First, let's start thinking about things that can happen to jazz it up. It's a wintery mountain, so attacks by monsters and avalanches both spring to mind.
For monsters, I'll throw in a pack of half a dozen wolves (Level 5 critters) using their stats as normal, so I'll take 60 SP out of the challenge to budget for them, making them 10 HP apiece. And, actually, I'll take another 10 to make the leader of the pack a grizzled, scarred old alpha with 20hp and +1s across the board (a poor man's elite).
Now, I could do the Avalanche by making it an attack the storm makes (effectively an encounter power), but I want it to have a little more substance, so it's going to be it's own problem. I'm going to take another 40sp from the budget for it. That's low enough that it's not going to be too tough to overcome, but it's not trivial.
AC: - Fortitude: 21, Reflex: 18, Will: 19
Wall of Snow (Standard, At Will)
+11 vs Reflex, 2d8+4 damage and target is Scattered
"Scattered" is a special status, indicating someone's been separated from the group. On the plus side, they can no longer be attacked by the Avalanche. On the downside, if the wolves comes, no one can help you out. A scattered character cannot make any attacks against the storm. To remove the scattered status, a character (either the scattered character or another character who is not scattered) must make a nature check with a DC of 20 (effectively passing up a chance to attack the storm.
Attacking the Avalanche
Circumvent - Outrun it, or do something like jump from passing boulder to passing boulder (Acrobatics)
Manipulate - Ride it out! (Athletic or endurance)
Understand - Find Shelter! (Perception or nature - as with the storm, nature gets a damage bump.
Smash - No options
Powers - Using a power with a strong movement component or with a Wall component (something that might create shelter) counts as a skill use. If it has a damage component, then use that, but if not, then for an encounter power do 1d10+4 base, and for a daily, 2d8+4.
OK, I think that covers it. Does that seem playable?
1 - These defenses are about a point high for a monster of that level. That's intentional, since skills are going to be a bit higher than attacks at this level.