I'll add that all the little adapters below will only work if your bike has m10 size mounting posts (my Schwinn apparently has something smaller):
I ended up changing course and going with Box Three v-brakes, and they work great.
And here is the aforementioned Schwinn (a 1999 Schwinn S-40) but now with 27.5" wheels, just because I think it's a cool bike:
