Rays are nice but expensive. I always wanted some but they're illegal here in California, and I'm a goody goody law biding type of guy (with fish at least, haha). There's a countless number of possibilities to go with though. Just make sure whatever you add isn't more aggressive than the arowana or he might get picked at. Silvers can be pretty mean at times though, you have to play it by ear and see what your fish is going to be like. I've kept mine with peacock bass, bichirs, datnoids, all sorts of cat fish, leopard ctenopomas, clown loaches, ect.
Just research what you want to put in there before you buy it. Some fish grow at extremely different paces, such as clown loaches which grow really slowly. If you got some small loaches now, chances are the silver would out grow them and one day may look at them as a food source. I've had that problem before, fish lived together for over a year and then one day decided he was large enough to start eating my shoal of loaches.