Discus aren't very active huh? How many have you kept and on what are you basing this statement? Seems like all of the discus you have had experience with are ones in a LFS that are more than likely kept in groups of 6 in a 20H, not to mention that they are most likely diseased, stunted, and on their way out due to improper care. All of my discus readily greet me at the front of the tank whenever I walk past and if they are on the opposite end of the 100g tank they immediately swim to the other end of the tank closest to me to see what's going on. The will swim in front of the tank all lining up thinking they are getting food whenever someone walks by and I feed them at least 3 times per day. I wish I had a decent camera to show you what happens in that tank. There are often "skirmishes" between the fish and they have a well established pecking order that even I am familiar with and I know who the boss is in the tank.
Africans in general would not be suitable for the tank because they will make short work of the plants, and also because there are most likely numerous small tankmates such as tetras and small livebearers currently living in the setup.