just a simple blue or black background can look great. Or you could go for rocks or plants. You could paint the back of your tank with emulsion. my background has plants etc on, my tank isn't heavily planted tho. really depends on what you like.
I use black bin liners. Black highlights the fish's colours IMO, and makes the plants stand out. And I'm too mean to spend money on a background anyway.
My hospital tank has an inside mirror backing - I hate it!
I think just a blue or black ground or somthing like a planted one i allways go for the blue ones because most of my tanks are breeding ones so it dont really care. but in a desplay tanks it would have to be plnt background.