I'm still fairly new to this so the more experienced keepers may have even better suggestions.
My ducks are on a mix of micro clover, sand & river rock and straw. I have added some siberian kale in the favorite snoozing area now that all the green plants have been killed by frost. It has sprouted and seems to be happy but the ducks may feat on it before it grows to adult size plants.
The microclover is evergreen and they don't seem to be particularly interested in eating it. They will drill it up when the enclosure is wet.I mostly have it in place to encourage earthworm activity and better soil quality. We used clean fill dirt to fill in the natural depression that their pen is in.
In the wet spots (around the stock pond, the bathing water and the extra drinking water) i have a layer of fine sand overlayed with the larger tumbled river rocks.
I have a layer of straw in their favorite outside snoozing place. Now that winter is coming I've spread a layer of straw everyone in the enclosure. As leaves fall I'm going to mix in leaf litter.
I may also add in some shavings or pellet bedding in the wettest area near but i haven't decided yet.
I had the luxury of seeding the microclover while we were constructing the area so i didnt have to worry about fat duck feet trampling it.
If you want to seed something like that now it may be better to put it down and then put a layer of straw on top.