For something unexpected, and likely requiring some detective work (at least very observant characters), incorporate the Texas Rose Festival. Many surrounding towns (and even from other cities) attend this in Tyler every year in October. It's fairly tame as far as carnivals go, but still worth going. And someone could be dragged by a relative (who convinces the others to come, or calls them in later, perhaps after being affected and calling to talk as if under a spell that makes the librarian suspicious about this drastic life choice and retirement being announced over the phone, a "goodbye forever, I've found true love" in a dreamy, half-asleep voice), if nothing else.
Alternately, events before this lead to this as a climatic finale.
Someone attempted to cast a love spell on some roses for a sweetie. It didn't work. Exactly. But the roses that grew out of it created insane aphrodisiac effects, perhaps at the first person looked at, which can lead to crimes of passion among other things, but not like what people expect at all. It also affects supernatural creatures, perhaps even stronger on vampires if they bother to smell the flowers (even just looking for prey).
It could be local at first, which makes it easier to figure out, but find that one batch was sent to the Texas Rose Festival so that everyone could know his/her pain (or even intends to "get lucky" or impress others by so many falling in love with him/her, or even a demon). Violent chaos ensues (a literal War of the Roses?) that will ruin many innocent lives unless they can find the cursed roses first (without falling to the power of the roses themselves), or at least counter the effects.
Just for something different.
I suppose if your group is more about power gaming and steam venting that prefers more video game carnage, you could have a militant minotaur demon coven there to avenge one of their own against ranchers or a beef company. Has the potential to be hilarious (at least a lot of bad puns) as well as lots of boss fights.
Demon to incorporate (given his expertise with herbs, might even combine it with the one above):
en.wikipedia.org
Obviously, you can switch it around a bit if you're uncomfortable with using an actual demon of lore. It's not like the occult lore of BTVS matched that of our world perfectly.
Also, how aware is the world of the Slayers now? Are there bounties on them, legal and otherwise? Is this a secret that your characters must hold, or do they face pressure of others who would control them (or worse)? How would their families react? How would colleges react, especially if any are into sports? ('Course there has to be at least one mad scientist on the college faculty...) Seems like a lot of opportunity right there.
Also, if many "Awakened" there, an interesting question would be why were there so many potentials here (and why not detected by the First)? For those into mysteries, that could excite many of them to investigate. It's just more about what your players want out of it.
