Bill Watterson was once asked how someone should play Calvinball. His answer was "
It's pretty simple: you make up the rules as you go."
Calvinball must have a ball. That is without question. Otherwise it is not Calvinball that is being played but Calvinsomethingotherthanaball. It must also be a reasonable ball. You have to be able to actually play with the ball. Generally, a ball larger than the hands of the smallest player would not be eligible. Calvinball does not necessarily have to have Calvin in it, however. In fact whoever's playing can choose to name it after themselves if they'd like, but if someone asks if they're playing Calvinball, the players can choose to say yes or no if they so desire.
The real fun to Calvinball is making up new rules. Said rules are immediately canon to the game even and especially if they conflict with other rules that already exist, at which point clarifications and other rules must be put into play in order to make the rules work together none at all. Punishment for failure to successfully accomplish whatever the rules establish should be embarrassing or painful but not much so and they should also be fun and involve a lot of running around and breaking things that will get you in trouble with the parental units later.
What a shame it is we can't operate this way in the real world.