I like val or some of the taller crypts as background plants. Amazon swords also work well in pretty big tanks. What size is yours? Also, you could try a java moss wall. There are a couple methods.
One way to do it is to get a couple sheets of plastic mesh and sandwich the moss between them. Anchor this to the back wall (suction cups work well for this) and the java moss will eventually grow through the mesh to the point where all you see is moss.
Another way (which I prefer) is to silicone cork bark to the back wall of the tank. The java moss can them be pushed into the crevices in the cork and it will eventually anchor and spread. A pro of this method is that the bark looks more natural while the moss fills in (it even makes a nice background without moss) than the mesh does. Some cons are that you have to plan this into the initial setup (it would be a huge pain to add this to an existing tank) and also the cork contains a TON of tannins. I thought I soaked and boiled the cork forever the first time I set one of these up, but I still had a tank of tea for a couple months (the fish loved it-the rasboras wouldn't stop spawning). I'm actually setting up a tank with a cork background right now, I'm not sure if I'll be adding moss or not, but I'll be putting pics up in the next day or two of the setup so you can see what the cork background would look like.