One of the well-known variants is with actors. You can centre the Universe on any actor, and see how many steps it takes to get to any other actor, via their films. The original (but not in fact the best) centre was Kevin Bacon, as documented at
The Oracle of Bacon at Virginia, using data from the IMDB.
It works like this. If you
are Kevin Bacon, you have a Bacon Number of 0. If you were in a film with Kevin Bacon, you have a Bacon Number of 1. If you were not in a film with Kevin Bacon, but were in a film with someone who
was in a film with Kevin Bacon, you have a Bacon Number of 2. And so on.
It's actually quite hard to find high (but finite) Bacon Numbers, no matter how obscure you think the actor is. Fun for all the family.