Answer: Would you be able to construct some raised beds? That way, you could bring in some topsoil and fill the beds with whatever you want. As long as you only cover a small portion of the ground under the trees, you shouldn't harm them. (In general, you shouldn't cover shallow tree roots with soil -- they're near the surface because they need air.)
If you didn't want to build raised beds, some large containers (like half whiskey barrels) might deter people and look attractive.
Hope this helps!
Q&A Library Searching Tips