I agree with Darth Kur on S.O.D. Some funny shit there.
Other "fun" songs are some Kiske-era Helloween, stuff like "Dr Stein", "Rise and Fall" and other little blurbs and sound effects like right before the solo in "Future World". Very lighthearted, but still catchy.
And I'll agree on Venom. If I'm in a total Venom mood, just singing along with the hopelessly blasphemous lyrics gets my laughing. Some of Cronos' "end of song" chants are hilarious. Like:
"Bring with witch to the altar priest!" - Sacrifice
"We're gonna burn this place to the ground!" - Heaven's on Fire
"Are you damned in hell???!!!???" - At War With Satan
My god, bloody genius!