Kida's given good advice, but I'll just add a couple of things:
-Catastrophes don't necessarily have to be natural disasters! Think of things like a bank robbery or a murder mystery (even one of those 'dinner party' types with actors), and those can give people plenty to play around with. They will probably require more legwork from the staff, but I think playing those kinds of NPCs can be fun too!
-For a slice-of-life site, more lowkey things like balls and festivals, as you said, can work well, but you can expand beyond those too. What about hosting some sort of event at a resort? A carnival or circus coming to town? A scavenger hunt at an amusement park? The opening of an ice-skating rink for winter? Halloween costume party or haunted house? Cruises? Those are other settings that people tend to get excited by, so don't be afraid to just capitalize on the ""aesthetic"" aspect of those things and then letting people free to do their own stuff.
-Prompts and randomized pairings. I know these can be unpopular, but listen—you can do great things with prompts! The trick is to not write a generic prompt that doesn't give people any reason to talk or continue interacting. I had a pretty good reception when prompting things like "person is given a love letter not meant for them," or "person is let into the wrong hotel room," etc. The trick with prompts (and open threads in general imo) is not being afraid to make them weird or outlandish in such a way that people likely will have reason to continue interacting beyond two or three posts.
-Contests/competitions can work too, whether IC or OOC. I've seen sites that divided people up into teams and did a posting challenge to see which team posts more. I've seen advertising or graphics challenges. People can be rewarded by lowering post requirements for their next character, by giving a free pass for the next Activity Check, with pixels, etc. It just depends on what works for your site. IC competitions can be a fun way to build rivalries and to give characters an activity to do—on my own site we're going to try to run a race through a corn maze in the spirit of Halloween.
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