That fruit I picked up last week? They're guavas, not quinces. Easy to see how I made the mistake, though. It's not like they were clearly labeled.
So, guavas. A quick search shows them to be just about as useless as mangoes. It's all jams and fruit butter, smoothies and the like. I suppose it's guava sorbet, then. Guava/strawberry actually, I think, as the two guavas I've got left aren't quite enough on their own. Good; It's about time for another sorbet.