Weathering cars and locos:

http://www.modeltrainsweathered.com/

Roads, Driveways, Walkways:

Scribe the styrene with a hobby knife, then paint it "old concrete".  Use a dremel tool to grind away the driveway cutout.

Here are some general minimums for N scale:

Radius - 12" (I recommend 15")
Easement Length - 8"
Track Centers - 1.25"
S-curve Straight Section - 6"
Max Grade - 3%
Yard Turnout - No. 4
Branch Line Turnout - No. 5
Mainline Turnout - No. 6

(these pictures are some ideas of what I'm looking to do)

Making Ground Foam at Home:

I learned this basic concept from a long lost website. I modified it to make it easier and faster for myself. The prime ingredient is foam. The yellow kind found in couch cushions. I have never purchased a bit of it. I just drive the alleys in town looking for discarded furniture. One cushion should be a lifetime supply. The other things you need are a blender from a yard sell (this one was 50 cents) a pair of scissors, a bottle of green craft paint, a bottle of brown craft paint, a 6 inch or so colander, water, and an empty cardboard box.

So you put about a cup of water in the blender and turn it on to a very high speed. You always want to keep enough water in the blender to keep it stirring. Cut off small cubes, slivers of foam, the smaller the less mess you will make. I go for 1/2 inch max sizes. Big chunks make the blender puke.

Keep chopping in little pieces until the blender gets about 1/2 full. Add more water if needed to keep the circulation strong. That's what chops the foam up.

At this point it's time to color it. Add cheap green craft paint. Add some brown next to darken the color to suit. The foam will lighten some as it dries, so go a bit darker than you want. Black can also be used. If you want fall colors use oranges and reds.

Let it blend 5 or 10 min. It's the speed of the blender and keeping it moving that makes finer pieces more than time. Shut it down and scrape the clingers into the mix and run it some more. Then filter it through a colander. Save the liquid and use it for the seed of the next batch, saves paint.

When most of the liquid has drained throw the foam in a cardboard box and make another batch, and another... The box wicks off the excess water, keeps the foam contained while drying, and can be threw away when done. After you made enough clean your blender and leave the box alone for 12 hours or so then start stirring it with your fingers every 2 to 12 hours until dry. Putting a small fan or out in the sun speeds drying times.

Each batch will be slightly different in color. When mixed it looks better than the monotone factory colors to me. Here is some after drying for 3 days. Anyway, that's all there is to making ground foam. Do not ever use you wifes stuff to do this!