So think of this as being ready for next year!
When your doing a Christmas diorama you do not need to spend much or even make a lot of effort.
For walls you can use Christmas wrapping paper
Either blue tack it to a piece of foam core board or cut to size and spray glue. I would not recommend wallpaper paste for this in most cases the paper is not thick enough.
You can buy miniature trees that are already decorated or you can decorate your own. Decorated trees can be found on e.bay or doll house stores/websites. Also look out for them in DIY stores (that's where I found mine) These often come with light so can give a very authentic look. (Note: remember to remove the batteries before you pack lights away. I forgot a couple of sets and they now no longer work)
If you wait until very close or just after Christmas there is a good chance you can get mini trees at a discount. I brought most of mine for 50p (75 cents) What style and colour is up to you and some trees need no decoration. If you tree does require decorations.
Lights.... think those little battery lights
Tinsel..... Tinsel pipe cleaners are good as these can be shaped. Gift wrapping tinsel comes on a roll and this is small enough for mini trees.
Ornaments... Look for interesting mini ornaments on doll house sites. You can also make your own pretty easily. Look for tiny baubles in amongst your stores normal tree decorations. Bows are another great look (inexpensive too)
Tree Toppers.... now these can be a little difficult. Miniature fairy's for hanging on a human tree. I found a box of Christmas star shaped baubles. These could be painted to match your tree if needed. OR you could always just make one ... cut out a star shape from silver/gold whatever piece of card and blue tack/tape/glue to the top of your tree.
Santa's found on ebay doll house section
Fairy is a little big ...but so what?
More doll house ornaments
Room decorations..... Lights and tinsel as above. Look out for mini ornaments for human trees or from doll house sites. Also look for cake toppers these are often a good size.
Those little mini reindeer can look good along with a whole host of mini items found in Christmas departments