I was also interested!

The biggest consideration is durability and flooring should be non-slip. I’d select the flooring first, which would dictate the vanity style. Shiplap could be a good choice on walls. Hard to be specific, without knowing more details, but if you search “cabana with bathroom“ on Pinterest, you’ll see lots of ideas perhaps tailored to your needs and style. If there is room, hooks for wet towels would be useful. If cabana is large enough, adding a shower would be useful (or add outside).