Having religiously watched old Gumby shorts on VHS in my childhood, I recently got the complete 1950s collection of shorts on DVD. And as much as I love Art Clokey's work, man, are they ripe for riffing. The "Robot Rumpus" short that appeared in an MST3K episode is just the tip of the iceberg.
They're a visual feast, but offer lots of space in the dialogue for riffing. And there's plenty of 1950s casual racism and good old-fashioned nightmare fuel to lampoon.
I particularly recommend "The Mocking Monkey," "The Magic Wand," and "In the Dough."