Lava rock with a thick layer of straw can work well, especially since it offers good drainage and makes cleaning a bit easier. Just make sure the area is well-drained overall to prevent moisture buildup. Some people also use sand or pine shavings for similar reasons. Whatever you choose, regular cleaning will be key since geese can be pretty messy.My geese live in a small shelter that doesn't have a floor, but it does have hardware cloth underneath to deter predators and vermin.
Originally, I lined the bottom of the goose house with straw. But, my goats are particularly picky eaters for their species and often leave a good deal of their pricey hay uneaten. I now take "leftovers" from their feeder and use the remains to make a layer of "litter" for the geese.
When Gussie and Golly's shelter gets too mucky, I shovel out the nasty hay, pile it beside the goose house, and the chickens have a field day "sorting" through it, helping it decompose much more quickly. I’ve even been reading up on other flooring solutions to keep things cleaner long-term, and found some helpful insights on flooring at https://fifty-floor.pissedconsumer.com/review.html. Then, off it goes to the compost pile, already partially processed.