Drew
There is nothing better then what Nature offers you. A small walk in your back yard would supply you with a big load of really useable twigs and other thingys. Also things like sand found in your backyard would be quite suitable. Another thing I use a lot for my sand is birdcage sand It has a really nice fine grain and will look very good on the dio.
Kittylitter is a nice supply for small rocks and pebbles and crushed it serves quite good as sand too.
What I usually do when I make my groundworks is the following. I built up all the leveling I want with the Blue Styrofoam. Next I spray/smear a diluted white glue mixture on it. Then I will add rocks (kitty litter) here and there and I will add my sand. I make a depression in the Foam where I will stick and glue my trees (branches found in the backyard) in. I also make holes where my tall grass (brush bristles of those big soft haired paintbrushes the DIY shop sells) will be.`
When all this stuff is done I give the whole base an Airbrush job in a dark brown color and after that I give it a lot of oversprays in different tones of brown (always lighter) till almost sand color. Then I add the long grass and I spray/smear more diluted white glue on several places for my static grass wich I sprinkle on it after the diluted white glue.
Then I drybrush several spots on the groundwork such as roads in light (sand) colors to accentuate the places where the wheels will hit the road more and I use darker colors for gras where a lot of people have walked on and in shades of trees and bushes.
Bushes can be made with the stuff called "lychen" its very popular amongst the railroad builders and it is a good base for different foliage. Buy some dried herbs like parsley spray your lychen with diluted white glue and sprinkle the parsley on top of it you will see that the result is an outstanding bush or even tree top. Really easy to make and not expensive.
In stores like Michaels or Wallmart you should be able to find more stuff to make your foliage even better.
Water is always a bit of a hard object to re-create but I have seen a guy working with realistic water from woodland scenics and that was actually really good stuff. You might be able to find it in railroad hobby shops. It comes in a bottle. It`s transparent, not heat generating like clear resin it takes a while to cure but you can work on it for a long time
These are my 2 cents for now