Floors for Gaming and Diorama Displays
Here is an example of an easy, realistic looking floor. Old and grey, slightly lifting, wooden planking; can't get better than that.
Made from coffee stir sticks, cut at various lengths and glued down with normal white glue. Note: Don't get glue on the top of the planks; it will give you grief later. Also, if you don't want them to lift and warp - after gluing,weigh them down with a heavy flat object overnight (Use a layer of wax paper or plastic wrap to prevent excess glue from sticking to the heavy object)
I stain them weathered grey using a darkish paint wash (i.e. from the cup you've been washing your brushes in - a perfect dirty gray). If you do get glue on top of the planks while gluing them down sprinkle fine flocking/dust on those spots while the glue is still wet or wipe off as best you can and then let dry and sand them with grit paper.
I like using coffee stir sticks for wooden planking. They also work great for floor joists and million other miniature building projects. Here are some 25mm examples using them in rubbled buildings. If you can't find boxes (1,000) of them in your area send me an email and I will get some for you. Hobby stores, craft shops, some dollar stores (pound shops), and restaurant\institution suppliers like Costco and The Grocery People have them.
For larger scale uses popsicle sticks work fine and for smaller scales I scribe them or split them with a steel edge and hobby knife. (similar to making planking on little doors)
The other flooring method I use is identical to the method used for making brick walls. Simple cut your floor tile pattern into the foam board and go over it with a pencil to deepen and widen the lines. Alternatively if you cut squares of card stock and glue them to the base you can get lifted tiles. Or, use clay, hydrocal, etc. and scribe the tiles in while it is still slightly damp.
Further on the floor
I used 1/4" thick textured ceiling/wall board and painted like stone marble.