If there's mud, the straw will get worked into it and become a soggy, muddy mess - make sure you buy a metal pitchfork... On the plus side, you can make bricks!
1/2 inch gravel works nicely, it packs down pretty well (you can buy a tamper at any hardware store), drains, is an easy surface to scoop and doesn't 'go back to the river' like river rock will (I swear that stuff floats!) We just had 19 tons dropped on our driveway/parking area - cost $200.
I know that you believe you understand what you think I said, but Iím not sure you realize that what you heard is not what I meant. -Robert McCloskey