Cohen the Barbarian fought the Luggage with his bare hands.
And it turned out a draw. It's hard to say which of the two was more startled by that.
Rincewind beating up a creature or creatures inhabiting the body of possibly the Disc's strongest wizard with his bare hands. And feet. And teeth, knees and elbows.
Twoflower helping out Rincewind against that same monster.
Twoflower convincing Rincewind not to let go of the ledge.
A minor one, but Twoflower showing that he isn't a total idiot by admitting that he knows Rincewind isn't a very good wizard was quite good.
Cohen's use of Obfuscating Stupidity to get them all away from some scary natives. "And now I stick the sword in your leg..."
It may be minor, but yet, seeing Rincewind actually succeeding in opening the closed door with magic (even if it takes all his energy) is pretty awesome, considering that he was a complete failure in magic.
The sheer stoic badassness with which Rincewind forces the Spell out of his mind. You. Out.