Ratatosk lives in the old world tree,
Merry old king of the Ash is he,
Laugh, Ratatosk, laugh, Ratatosk,
Gay your life must be

As a wise man once said, "I must obey the inscrutable exhortations of my soul." Don't judge me.

* Someone has been stealing the "enormous, sterile seed pods of Yggdrasil" that the ratatosk collect. They're unable to track down the thief, and need to implore outside help such as the PCs... of course, the question of why they're being stolen is a mystery that might intrigue certain individuals as well.
* A venerable silver-furred ratatosk knows the information the PCs seek, and his price is fairly simple: a riddle he hasn't heard before. Or a good insult. Do you have any of those?
* It's a cold and dark winter's night on Yggdrasil, and the PCs set up camp. Unfortunately, the local ratatosk tribe takes very poorly to even the most carefully controlled fires lit on the world-tree. Getting a good night's sleep will be complicated.