Roll a rock up a hill
Sounds too easy. It isn't. You could probably replace the word "Rock" with "Great big 2 tonne boulder". And the word "Hill" with "Mountain". Not so easy now.
In principle, it is a simple task and requires very little brainpower, which is a huge relief for our heroes, who couldn't exactly be described as intelligent. Or smart. Or even dumb for that matter. Anyway, it is quite simply moving a specified (by the quest setter) lump of rock up a designated mountain. There are however a few dangers with this - mainly heroes being crushed by their boulder. Also, there are usually many monsters on said "hill", who are doubly annoying because not only do they try to kill the hero, they also force the hero to stop holding the rock, allowing it to roll back down the mountain. Despite these minor setbacks, this quest should be completed within 6-12 hours; this quest is in fact nothing to do with Sisyphus.
Top tip: If attacked, allow the "rock" to crush your opponent. It was going to roll downhill anyway, so you may as well use this to your advantage.