For me, it depends on the vampire much like any other sim. However, I have found after playing them for a while that coffins are your friend. Vampires don't need to sleep but there is nothing preventing them from hibernating the day away in their coffins. So, if they aren't actively working on anything, I usually have them hibernate until nightfall and then head out for dancing and "drinks."
When I do have them actively working on something, it's usually art, music, reading or chess. When I play the Vatores they end up playing a lot of chess.
I imagine that this is an "in character" problem for vampires. They're immortal. They have a lot of time of their hands. So, for those who are active, I imagine then tend to obsess about something, master it and then move on to the next obsession. So, pick a skill or aspiration and work on mastering it... then pick the next one.