I used gravel over landscape fabric for the weed control myself. I've seen wood chip mulch, I've seen grass. It all works to varying degrees, so you just need to decide what you like the look of, and how much effort after the fact you want to put into it.

Remember, design is the fun part!

Here is a link that might be useful: Sinfonian's garden adventure

I think it depends how deep you need to fill. If you're only talking a few inches, go with a mulch of some sort (I'm partial to wood mulch myself, but that's a personal opinion). If you have to add more depth, I think I'd use dirt then top it with one of the mulches.

I'm totally making this up, having no experience, but I'd think mulch has a tendency to stay damper longer....I'm thinking haven for bugs.

Then again, like I said, I totally just made that up.
Sounds good.
But I've got nothing to go on to support it.