When it comes to fish "exciting" is kind of hard to find, but here's a couple of fish I find fun.
An Oscar! I have one and they are awesome. They eat from your hand, you can feed them anything! I feed mine insects, feeder fish, flakes, pellets, mice. They also always come and greet you when you go to the tank. I can actually pet mine. But only 1 would fit in a 55gal. (55gal would be the absolute minimum for as oscar as they get a good size.)
You could also get a couple piranha. I've never had any, but from what I've seen and heard, they are pretty exciting to watch, especially when you throw some meat in the tank. You can feed them anything too. Mice, frogs, fish.
