If you find some driftwood of the general shape of a tree, you just tie moss onto it and keep it trimmed. If you don't keep it trimmed, it looks ugly. You could do one of those, and then have a kind of "plains" hardscape, and then add some smaller trees around. Smaller trees are pretty easy, you just take bamboo skewers, cut them to size, and then tie moss onto them. If you keep them properly trimmed they look like evergreens.