I've had most success with skill challenges when I think of them as specific types of
challenges (omit the "skill" part), not a one size fits all mechanic. By
specific I mean NOT excessively abstracted, which was often the case with 4e's implementation.
With that in mind, let's look at your overall situation & each of your proposed challenges.
Here's the setup: the PCs will be traveling overland from Cragmaw Castle to find Wave Echo Cave, led by Reidoth the druid (NPC). It's roughly 8 to 10 hexes away, 40-50 miles. I normally roll random encounters, but I want to eschew that idea and instead do THREE challenges: Easy, Medium and Hard (DCs 10, 15 and 20 respectively). I thought I would do a social encounter, a hazard encounter and a combat encounter (Easy, Medium and Hard respectively).
The overall situation is that you want to run a resource attrition scenario during their 3-days travel. The challenges you've written may foreshadow things to come, but they seem primarily concerned with stripping the PCs of gear, Hit Dice, and/or imposing Exhaustion. Be aware that many modern players tire and/or get frustrated quickly with resource attrition scenarios; one way to ameliorate this is to introduce opportunities for them to gain something (information, gear, blessings) during their travel.
Easy (social) The PCs are near the Triboar Trail and camping for the night when a group of travelers join them. The travelers are actually bandits and will lull the PCs into trusting them so they can be robbed, pickpocketed, etc. (4 successes before 3 failures). How are some ways that this can play out in real time as the players use skill rolls? I am VERY rusty at challenges and forgot how to make these things flow well.
You need to establish what those 4 successes and 3 failures represent exactly – too often skill challenges break down because they lack specific context.
Sounds like the successes should be incremental hints that the travelers are actually disguised bandits. The couple hints should NOT immediately scream "they're bandits"; instead, there should be multiple possible explanations that get whittled down as the players gain more hints. For instance, shoddy scavenged gear with multiple villages/nations of origin could be explained if the "travelers" are nomadic, pilgrims of multiple ethnicities, former hirelings of a mercenary party, or they found/stole the gear.
What do the failures represent? And what does failing the challenge mean? Does failing the challenge mean that everyone turns in for the night unawares, and the bandits posing as travelers rob the PCs during the middle of the night, so the PCs awake to a bunch of missing gear and no sign of the bandits? What about the sentry the PCs posted, the familiar keeping watch, the
alarm spell the wizard cast, etc? What if the PCs decide to follow the tracks and ride after the bandits (VERY likely given player psychology)?
Medium (hazard): Flashflood in the foothills. A torrential rainstorm fills a canyon and the PCs have to navigate to safety or lose HD or gear or gain Exhaustion or a combination. (6 successes before 3 failures). I guess this will mostly end up being physical challenges for the players, but I could use some examples of how others have done this.
I'm not entirely convinced about on this one. Anyone trained in survival (let alone a druid casting
druidcraft) knows how to recognize signs of an imminent flash flood and that you want to get to higher ground ASAP. It's a very specific situation when a group of travel-hardened adventurers would be at risk from a flash flood. One of the obvious solutions is to get to high ground and hold position to wait it out, which is what I'd assume most parties would do unless there's a time pressure driving them forward.
Regarding using Exhaustion as a penalty for failing in a skill challenge... My experience is that players are extremely adverse to accumulating Exhaustion (many refer to it as a "death spiral") and will do everything in their power to get rid of it. Typically, this means long resting until no PCs have exhaustion.
Where I think the idea of a flash flood works better is a pressure that drives exploration. For instance, say we're following a river canyon when the ranger notices early signs of an impending storm – birds seeking shelter and being very active preparing, a sudden calm, gloomy clouds in the distance, etc. The PCs are now driven to seek higher ground. What is at higher ground? Which way do they go? Does the ranger notice the impending storm while the party is split up (e.g. some are portaging the canoe or hunting)?
Hard (Combat): In the mountains, not far from the entrance to Wave Echo, they stumble across Mondo the Hill Giant. He's big, he's stupid, he's an naughty word, and CAN kill the 4th level party pretty easy. (8 successes before 3 failures). This would only devolve to combat if they fail the challenge, OR I can knock them down HD automatically and say they escaped, or it takes days to circumvent Mondo and they gain Exhaustion by having to climb steep alternate routes.
Mmm, I'm not convinced a CR 5 hill giant is a death sentence for a 4th level party. What are you envisioning here? Is there a cowardly hill giant high up in the mountains who is throwing boulders down? So the challenge is an ascent while maintaining cover / staying hidden from the hill giant to avoid risking getting hit by a boulder? But once they're in melee with the hill giant, he has no interest in a fight? That
seems to be what you're aiming for.
If so, then I'd probably handle it that each PC needs to make a successful check (or cast a spell that circumvents) to reach Mondo's position. Each round any failed check means that PC is temporarily exposed, and Mondo will throw a boulder at one of the exposed PCs that round. I'd look closer at average party HP and average hill giant boulder damage to determine whether to use the hill giant stats for the attack or derive my own numbers. Once everyone succeeds, the entire party reaches Mondo's position and the hill giant is no longer hostile. Of course...the players may very well be feeling hostile at that point so it could devolve to combat if you have some feisty players...
Notice that they succeed or fail as a group, and there's no "the wizard
dimension doors first" or "the ranger reaches the ledge before the rest of the party." This is to avoid framing the situation as a normal combat.