To achieve more enjoyment from your favourite track many people decide to fix the track to a board and decorate it with buildings, obstacles, garages and pit crews, track side advertising, and landscaping. You can find above links to their respective topics, with descriptions and evaluation of different methods of scenery construction:

My advice is to read all the methods used and decide for yourself which one fits your circumstances best. As what may have been right for me, might not be right for you.

Scenery can be bought or built, expensive or cheap. It all depends on your skills, the amount of spare time you have to spend on your project, and your budget. Ready made buildings and structures are great for when you don't have the time or skill to create your own. They are expensive however especially if you buy good looking models (the ones without the plastic look to them).

When it comes to vegetation you can buy trees, but you will still have to do all the grass and hill work, as ready made alternatives just don't exist. Landscaping is the easiest and most fun stuff to do. If you are building track for you kids, this is often the part of the project which you and them can work together on. Working with papier-mâché and model-rock is great.

I would like to repeat my self, read all the methods used and decide for yourself which one fits your circumstances best. If you haven't got the money, build it. If you haven't got the time, buy it.

2010 Update

To aid in creating a realistic scenery for your Scalextric race track there are now available landscape mats and scatters.  There are also scatter papers which can be made into hills.