I do have to apologize to luscious_purple. The event's momentum seemed to naturally carry from dancing, to a carol-dance, into caroling, and she didn't get to perform the two instrumental pieces I know she was working really hard on. And she was heralding court, so we couldn't even really do them there. :( If we can mutually make the Midwinter's Revel at the end of the month (we have a family baptism; if it's in the morning I hope we can still go) maybe we can do them there?
I started thinking that we could do this kind of thing more! more! more! but a reality check has been cashed. This event was unique in that:
1) It was held indoors, in a single room, with no scheduled activities other than dancing
2) Singing carols is also a modern Christmas tradition
3) People already knew most of the carols (see #2)
So... I'm thinking maypole. Work with me here.
Spring events are usually fighter-centric and outdoors, but there's usually a population of non-fighters who would appreciate something to do. A maypole dance could go 30-60 min and be considered unusual enough to be mentioned in the event announcement. This goes to addressing point #1 (creating a space where people have no reason not to participate).
I can't think of any generally-known spring carols off the top of my head, but it occurs to me that "Now is the month of maying" is pretty darn well-known and would lend itself to repeat-after-me plus fa-la-las. (I mean, the verses get repeated anyway, right? We're just putting the repeats in a different place...) A maypole can be danced just by skipping, which people know how to do. This goes to addressing point #3 (you have to work with what people already know to get them to widely participate).
Maypoling is a "known of" tradition, even if it isn't done much outside of DC Revels spring events and possibly the Ren Faire. People might hear of it and get warm fuzzy holiday associations, which goes to addressing point #2 (that it seems like a natural and desirable thing to do at the time).
After the maypole is danced... I dunno. Some singing and dancing, I hope. Or games! Back in college, we used to play a variety of medieval (or medievaloid? I don't even know) outdoor games, like blind man's bluff. That could be fun, and very kid-friendly.
There's the physical plant problem of actually constructing a tall pole (that breaks down for transport) plus a weighted base of some kind to hold it aloft. Not to be sneezed at, but should be doable.
It's something to think about, anyway.