Pea gravel is working very well for me. I can hose it off easily when needed.
They do not have to have sand, but some believe it is the easiest bedding. I do not like sand that much because it has no organic carbon in it. The carbon seems to help keep the poop from getting as nasty between cleanings.
(I am a compost nut and biologist, so please bear with me here.)
Anyway, I use some sawdust (pelleted is what I can easily get) to cover their run, except under and around their water pans, where the pea gravel is. The sawdust soaks up moisture from their droppings. I can lightly rake off the little piles which then can go on the garden, or I can just let it work in for a while.
Depending on the density of the run (how many ducks per square foot), you may or may not be able to keep grass growing. Even though they are small, ducks seem to be able to flatten an area pretty well, especially after a rain.
Since everyone manages their ducks differently, I would say you could try just leaving their waste where they put it (grass loves it) and see how it goes for a week or two. My night pen has sand in the bottom, and when I had sand only, the nitrogen in the droppings made for a whiffy pen. When I started adding a layer of sawdust (which I change out every few weeks and add to the compost pile), the smell went away.
