So, so, SO many. Nearly everything, including Hamel transforming into a Mazoku has been played for laughs.
The very first chapter gives us a beautiful pearl when Hamel uses his music to turn the evil demon who killed Viola's mother into his surrogate mother, complete with sparkling tears and matronly clothes.
Turns into a Brick Joke many, many volumes later, when Viola shows up to fight in the final battle while riding on the back of his new mother. (He's also dating the flower girl from the fourth chapter.)
Volume 16's unforgettable scene in which Hamel, Raiel and Trom show up to talk to the king of Score, in front of the whole army. Hamel's demon blood had taken over in the middle of a ball (hosted by the king) in the previous volume, and he'd been called to a hearing after the ensuing destruction. It's such a heavy, serious moment, that he actually starts to apologize to and thank his friends (something he'd never done) before walking into the audience room... then says he was thinking about wearing something cheerier in order to look harmless and friendly. His worried friends immediately offer to wear the same thing as moral support. Of course, the friendly, harmless-looking clothes turn out to be sequined leotards with feather tails. Then, of course, Hamel went berserker again while wearing that outfit... in front of a room full of soldiers and a King. Hilarity Ensues.
Then there's the revenge of the orphaned deers in chapter 117. A pair of deers whose mother had been (accidentally) killed by Hamel's flying violin (don't ask) managed to somehow bury their mother and raise a mark, then cry in front of it for several pages, swearing revenge on that evil black-hatted man. When they start to wonder how to find him, the heroes just happen to show up nearby. Then Hamel decides he's hungry, grabs a stick from the ground, shuffles into the bushes, and comes back dragging the two (now dead) deers' carcasses. The group happily eats them in a donabe.
"And so, the young deer met their mother in heaven. They are now together, forever."
The "hot springs chapter". The group stumbles upon a hot springs after weeks of travelling. Hamel and Trom Bone are, of course, extremely excited by the chance to peep at the girls. They hide behind some bushes at the edge of the spring, commenting on the slender figure they can see but can't identify in the mist... and which turns out to be Raiel. They are NOT happy with him, and poor Raiel can't understand why. Eventually, they DO get to peep on Flute and Sizer, who were bathing in an extremely fanservice-y way, complete with Skinship Groping. They were Mazoku in disguise, hoping to take the heroes off-guard. In a fit of rage, amid tears of blood, Hamel and Trom beat the crap out of them, complete with shoving swords up their asses. It has to be seen to be believed.
Then there's the volume extra, depicting the "training" the mazoku had to go through to impersonate a couple of bathing girls being extremely girly. Brain Bleach is required.