Geoff,
I think it really depends on how "handy" you are working with wood. If you know what you are doing, then I would use a veneer covered wood and make your vivarium out of that. Veneer is great to work with because you can get it in many types of wood and then there are many different ways to stain and finish it to match your furniture.
If you are looking for a more natural wood finish then do not use plywood. Plywood is just a "hodge podge" of woods that have been glued and pressed together. This makes it very difficult to finish because of all the adhesive properties in it.
Melamine is a great product to work with because it doesn't need to be finished and cuts easily. You can get melamine in different colors and fake wood grains. With that said, the wood grains are harder to find. Most stores will only carry white or tan.
Good luck....