If Spike played 'The Sims':
The house he designed would be extremely nicely decorated, with the wallpaper and carpets and furniture all matching. He'd invest lots of time and energy into giving all his characters fulfilling, active lives, and especially making sure they were all paired off happily in romantic relationships. Especially that cute blonde female character he designed and her platinum-haired boyfriend who is in no way a self-insert character, not at all.
If Buffy played 'The Sims':
The house would be very striking and imaginatively decorated, but there'd be rough bits where she got impatient and just jammed things together. Her characters would manage okay for a while, but slowly get more and more dysfunctional as their needs went unmet. Eventually she'd give up and ask Willow to help sort things out. (She asked Dawn once, but then saw what Dawn did to her own characters and freaked out.)
If Willow played 'The Sims':
She'd build a very basic house and mess around for a while trying to get two female characters to pair off romantically. Then she'd get bored with playing and start modding the game instead, designing her own furniture and trying to hack into the character scripts to get them to perform actions they weren't programmed to do.
If Dawn played 'The Sims':
She'd design a bizarre house with every possible item of furniture crammed in and let her characters live normally in it for a while to see what happened. Then she'd deliberately try to set them on fire or make them starve to death or drive them mad.
If Xander played 'The Sims':
No, he didn't spend an hour searching online for an anatomically-correct nude cheat code after Willow mentioned off-hand that such a thing existed. Honestly.
So - what about the other characters I've not mentioned? Or are there any other computer games that these people would bring their own unique touch to? Let me know in the comments!