Cardinals, Glowlight tetras, Black neons...they're all good and can tolerate the higher temps like discus. Cardinals are usually the best due to their striking colours and they move slowly like discus and eat food in the mid-region of the tank. I've got a golden nugget in with my show discus tank and he looks amazing against the discus colours. As I turn around I see him out now hehe.
Include some driftwood if you can. This is more common in their natural habitat, they like to hang out in the roots of fallen trees. There is the odd plant. The water is a cross between blackwater and clearwater. The key is to keep the water conditions steady.
Hope this helps