I've tried to keep them as cheap as possible. If you have access to more expensive and durable materials - use them. Spaceships from supermarket boxes and radio sets from discarded stationery presentation boxes and old, empty ballpoint pens. Decoration can be achieved by the use of leftover household paint, sticky tape or thin insulation tape. If you have access to a computer, a bewildering array of typefaces are available for you to print out miniature notices to enhance your creations.
Some basic principles have to be observed here.
Safety is paramount, especially where you are making these things for children to play with. Waterproof tape is useful for covering places where water might seep in and can be covered over with paint to provide a proper water tight joint. Craft Knives are ideal but must be treated with caution for the same reason. Wheels can be made from multiple layers of cardboard, cut to shape or by using tightly packed cardboard tubing, suitably cut to size. I hope that you have been inspired by the above ideas.