In addition to what people have suggested....
Lots of green and brown for dirt / grass.
Road base plates are nice for urban settings.
We often use those big elevated base plates like that which came with the city police station, pyramid, and various castles. They quickly add a lot of vertical diversity with out using bricks and can be made into epic forts of osum.
If you are near a Lego store with a Pick-a-Brick wall then stop by once a month. Sometimes you get lucky and they have a bin full of trees, plants, flowers, palms, and sometimes stuff that is expensive to get in bulk. You have to be vigilant though and not ready to pull the trigger when you go. Because several times I hesitated, came back a day later, and they were completely out. This happens with things that aren't commonly found on the wall. For example they always have 2x4 bricks but if there is a bin of bamboo plants it will be gone in a day or two and you don't know when, or if, it will be restocked.