Trouble

Simple enough, trouble, but it comes in so many forms. In order to get to the Upper Valley, the party has three choices. They can take the long road east and make the precarious journey up the outside of the cliff, they can fall in line and make their way up through the cave system on foot, or they can negotiate passage up the cliff face on the next lift.
The caves can be navigated without guidance in a six hour span. There is always a chance of delay with other groups trying to move through the same spaces, but generally these are only short delays. Getting lost of off track is rare, but given that there are a large number of caves, some natural and others excavated, that have been connected to allow travel up through the cliff face, it is still a possibility. Regular survival checks can be made to stay on track (DC12). Failure just adds an hour to the travel time, to allow for correcting the route.
There is a danger of attack along the way. Since many of these caves are natural, or connect to natural caves, there is an opportunity to encounter various cave dwelling creatures. The most common groups encountered are bands of troglodytes, other than creature denizens, which are abundant (spiders, bats, etc.). Since the route is fairly regularly traveled, the time of day matters. Creatures like spiders can be encountered at any time, but the humanoid creatures tend to stay hidden during the day when more people are on the road. They like to attack at night, and prefer to target isolated wagons.
The party is more likely to hear signs of a scuffle and be in a position to come to someone's aid, since their gear and lack of goods make them less of a target to the attackers. If they are accompanying a group of traders, ignore the last point.
If the characters choose to negotiate passage on the lifts, the journey takes 8 hours, and may require additional time waiting while supplies are unloaded and loaded. While waiting, they can take a rest, or they may be asked to assist in defending against any close skirmishes. Once underway, the journey is mostly benign, although an equipment malfunction might cause a delay. Another possibility here would be a work stoppage in protest of some issue by the lift crew, or an attack against them by a hostile force that caused them to halt the process. In this case, the players may be interested in getting off the lift in order to assist, which could take some thought and ingenuity, not to mention a series of skill checks, I'm sure.
I don't really anticipate anyone taking the long way, but it is a treacherous route up and through mountainous terrain - ettins, giants, rockslides or avalanches, and giant bird or dragon attacks would all fit nicely.