I second the background. Even if you don't like any of the particular backgrounds they have for sale, even a plain black background will help calm your fish. It also makes whatever you have in the tank - plants, rocks, driftwood, fish, etc - stand out more clearly. Finally, it hides whatever's on the other side of the tank (cords, filters, electrical outlets) that make it clear that it's a fish tank in a house. The black background will help create the illusion that what you're looking at is a little slice of nature and not just a fish tank.
More rocks is always better! You can make some really interesting caves and hiding places out of rocks. Some of the bigger fish stores will sell rocks for aquarium use, but you can usually do a lot better if you buy the rocks from a landscaping/gardening center or find them on your own. Just stack them up and make some caves. You can use aquarium sealant to glue them together if you plan on keeping rowdy fish that could knock them over. Here's a good article on what kind of rocks are appropriate for the home aquarium (but don't boil your rocks!):