If you check out this
Jonathan Coulton blog entry, he lists a large number of great easy covers of hard stuff - I really like
Nina Gordon's Strait Outta Compton.
coming back in with more thoughts...
What part of this appeals to you? I have a huge number of genre-bending covers. If it's the barbershop, there's Metal Shop by Da Vinci's Notebook (classic metal song medley). And don't forget the eleven polka medleys that Weird Al Yankovic has done over his career so far.