Another nod for Severums. More active, social, and certainly more interesting to watch than many of the other S Amer Cichlids. Severums will also tolerate other species in a community tank as well.
Firemouths tend to be active and spend a lot of time foraging. Many of the other big cichlids like Dempseys, Oscars, etc., tend to just get big and sit around doing nothing.
An Angel tank is more sedate, but looks nice.
If you want 'interesting' and 'active' you can't beat an African/Mbuna tank.