Grease says is covers a solid surface with a slippery surface...So what is the ruling for mud, swamp and grass if this spell is cast?
What are some other creative uses for Grease spell?
Yes. From a thread about a web anchored to flying creatures, a solid surface is defined as a surface that doesn't move. Any type of ground works.
It's useful for anything that requires lubrication. I'm sure that the good folks in the Succubus in a Grapple thread will help you with some of that. For more family-friendly uses, I've had a few PCs use it to convert a broken gutter into a slide, shove a bulette into a tunnel it couldn't get out of, and the like.
One note, though- it creates slippery goo, not actual grease, and even if it were actual grease, grease doesn't really burn as well as many people think. Setting it on fire doesn't really work.
try out my thead... Grease-spell-ideas