A 90g would make for a very nice discus tank. You could fit in probably 6 or 8 fully grown adults, and I'd try to get 1 male or 2 males and 5 or 8 females. Should provide lots of pots and cover for them, and breeding shouldn't be a problem.
Good luck, they're a time intensive fish but well worth it in my opinion, they're simply beautiful.
As for plants, I'd stick with the old standby - Anubias, Amazon Swords, some java moss, and a decent amount of floating sprites to diffuse your lights. If you're not into plants you could keep a barebottom tank; it makes maintenance much easier.
Good tankmates for them are corydoras, silver hatchets and several of the non-aggressive tetras - serpae's, cardinals, flames.