The beauty of public domain cartoons is that they come from a time when pop culture was bat-shit insane - and I don't mean the calculated, "trying to damn hard", imitation insane we get so much of these days.
BALLOON LAND (1935)
Here's a great demi-plane to introduce your characters to, especially if they need something from the inhabitants that they cannot get by means of violence. And wouldn't you love to pit them against the Pincushion Man.
CANDY TOWN (1931 or 1933)
Another odd demi-plane to hide behind the next magic mirror in your camapaign.
This is one of the great weird cartoons ever made. Talk about your mythic underworld!
I've always thought a campaign world where this sort of thing determined the season would be a fun one to challenge players.
THE SUNSHINE MAKERS (1935)
The distilled sunlight potions are excellent, especially if you have a stereotypically dour dwarf in the party.
TO SPRING (1936)
Another in the vein of Summertime, this one with subterranean elves who color the flowers in the springtime.