What can players visiting via Dodo Code do to my island in Animal Crossing New Horizons?

Pick up items that are dropped (not placed), pick flowers and harvest trees (which isn‘t that „bad“ bc this will grow back, since you need an axe/shovel to completely kill/steal flowers or trees and you can only use the axe or shovel when you are Best Friends with the Person.

So actually nothing that doesn‘t grow back, just make sure you don‘t have items laying around. Same goes for money trees, fence them around. I always walk around my island before opening with dodo code for random players, I pick up the message in the bottle and other stuff so I don‘t have to worry about something