I seem to remember seeing a sort of more 'modern art' version of the Mona Lisa somewhere... I may be way off, but maybe hanging in the Count's Castle in one of the DVDs or something?
The "Sing your Synonyms" sketch I remember pretty well, though I don't completely remember the lyrics. I know it was based off the song "Let's Call the Whole Thing Off" (by George and Ira Gershwin, apparently first from a Fred Astaire movie). Bob isn't just holding a granite ball, he's supposed to be Atlas, the Greek god/hero who holds the world on his shoulders, so it's more like a granite globe. I seem to remember Elmo's face poking through a different painting similar to Telly's in the Mona Lisa, but I'm not totally sure on that one.