Odd trees will always make for interesting landmarks. For example, a tree growing from another tree:
In the backyard of my childhood home was a single tree onto which my father had grafted plums, nectarines, and peaches. The oddness and wonder of how humans could manipulate plants has stuck with me. And if you've been on the internet long, you've most likely seen posts of trees shaped by people into chairs or woven patterns. These fascinate me as well. A braided tree could make for a memorable feature in your sandbox:
These trees always seemed very elf-like to me. And just think, what elves, living for centuries, could coax from nature with patience. Bridges:
That is in Bangladesh. Or this:
Of course those are crude in comparison to what elves would be able to craft. I'll leave the elf-bridges to your imagination. How about roadways lined by trees for miles, even across plains where trees are scarce. Or, within, forests, avenues of trees still discernible to the observant:
But, the truly fantastic would be whole buildings shaped from trees:
And even that is just the barest idea, crude in comparison to my imagined elf cathedrals. Even whole elvish cities shaped from trees only to be abandoned.
The last few pics came from here, look there for more interesting landart ideas.