Rosso doesn't need much exercising since he has a reflective corner that he likes. Every now and again though, I'll stick a clean finger in and he'll go after it like a shark. Rosso's kinda difficult... it's seems like he learns the tricks, except the finger trick.
Poseidon is new, so I'm trying to figure out what excites him. I still try to get his attention for a few mins each day.
Sid enjoys the chase of being cupped for water changes. He lets himself be sucked into the cup, and then leaps out as I raise it to bring it out of the tank. I thought he was stressing out, but he circles the tank and heads straight for the cup again. After one or two escapes, he allows himself to be caught with no effort. I give him a pellet, and then he's a happy camper.
I encourage him to flare once a day, briefly. He loves this, and strut-swims around for half an hour after (Demyx, on the other hand, hates it and gets stressy, as he does over pretty much everything so I avoid flaring him).
Mostly, I just provide places to explore and get 'lost' in, plants and caves/tunnels, as much as his tank space allows. He gets quite 'busy' patrolling all the nooks and crannies.
We also play 'hunt the food', which is fun with live foods. and he gets loads of 'hellos' from us all as we pass by. He never fails to leap out, all ninja-like, to say hi back. So I think he's pretty well entertained and excercised, as much living in a fish tank permits.