You can kickstart your custom activities from the area OnEnter event. I have quite a lot going on there in my module (including random appearances for part-based citizens) so it was a bit laggy at first, but I fine-tuned it to an acceptable level.
I seriously considered the off-the-shelf activity systems for Dark Energy. Ended up doing my own, but never found time to make it so feature-rich, so in hindsight I'd say go with a package.