Two Heads are Better than One

Of the overland encounters, the one that intrigues me the most (of those mentioned) is the ettin ambush on the scree-filled mountainside. This presents an interesting opportunity. Ettins are CR4, which shouldn't be a huge challenge for characters of this party's level. The difficulty of the terrain will be a good equalizer, but the strength of an ettin lies in their two damaging melee attacks.
Moving along the road is treacherous, but not especially difficult. The road runs fairly level across much of this ridge, eventually running down to the valley floor in a series of switchbacks. At this point, though, it is generally flat. What makes it treacherous is the dry, loose footing and the narrow track.
Much of the morning has been spent picking your way across the ridge, being careful not to slip down the seven hundred foot fall to the base of the rocks. Unfortunately, there has been a recent rockslide from somewhere higher up the hillside that has sent a pile of loose rocks skittering down across the roadway before you. Investigating the slide, you can see no safe way to get across the slide without clearing the road. This leaves you exposed and in danger of causing further slides from shifting the rocks.
High on the slope above, three ettins have emerged from their caves to watch the party's progress across the ridge. In a rare moment of consent, they all agreed to dislodge a few good-sized boulders which caused the hillside to cascade down and block the road. Perception DC25 to notice the ettins above, unless the characters actively look for something of that nature, in which case either drop the DC to 18, or give them advantage. I think if they have Survival as a skill, they should get advantage. If they actively look, lower the DC.
While the PCs work to clear the roadway, the ettins will shower them with rocks from above. They can be targeted in return from distance, or someone could attempt to climb up to them. From your vantage point, you can see that the ettins are near the upper limit of the slope. Above them, the cliffside is still intacts, and you can see several caves opening into them. The ettins are about 150 feet above the road. Climbing the hillside is difficult terrain, meaning all movement is halved. At the end of each move, the PCs need to make an Athletics DC18 saving throw to keep their footing. If they fail, they fall prone. If they fail by greater than 5, they slide down the hill 10 feet.
Once anyone starts to climb the slippery slope, two of the ettins will start down to battle them. They are more sure footed in this terrain out of familiarity, but more reckless about their descent, also. They can move down the slope in two turns, but have to make the same Athletics ST. The extra movement doesn't count as extra movement for them, allowing them to take an action on their turn as well. If they fail their ST, they can "ride the slide" for the extra 10 feet and not fall prone. They are eager to join the fight.
Once they are defeated, their caves can be searched for treasure. Among the discarded bones and scraps of armor scavenged from previous victims, there are a number of usable items still to be found. I'll have to figure out what at another time.