There are so many different answers/opinions on discus, but IMO: (BTW, this is al based on the chance that you are being grown out, adult fish, because if you are planning on buying juvi's, they really need to be grown out in a bare bottomed tank with daily w/c's).
substrate: sand. a light in colour sand if you have any pigeon based discus, as they will pepper if you are using a dark substrate. Also being cichlids, they often graze of the sand to eat. Mine eat more off the sand then they do from the water column.
tankmates: personally, I have mine with hatchetfish, tetras (cardinals & rummy nose), and a bristlenose pleco. I did have a few rams, but they passed away. Other good tank mates could be other dwarf cichlid species, ottos, pencil fish, corydoras, etc. I depends on what your temp will be at really, as not a lot of fish can handle discus temps. Some people have reported them eating some tetras, but larger, more wide-bodied species are usually okay. I did see one eat a cardinal of mine, but it was during feeding time and I think it just got over excited as it spit him out right away. The cardinal didn't make it though as there was a chuck taken out of him. That's the only incident I have experienced with mine that I have witnessed. They generally leave everything alone.
Decor: Plants and wood. They can be quite shy so they do like a place to hide sometimes if they get spooked. If you are going for a biotope type thing, you will see that most people use only wood, as a lot of their natural habitat only contains wood and not much plant life. I think that they look great in a planted tank and that is how I have mine.
Water type: Soft and acidic. A lot of tank bred discus are pretty hearty these days, but they need soft, acidic water in order to thrive. They might do okay in harder more alkaline water, but if they do, their life span will be significantly shorter. A lot of people use RO water if their water is on the harder side, whereas I don't because I have very soft water where I live. My tank ph is 6.0 and dKH is between 2-3.