Things vary so much by parts of the country I can't give you much advice.
If I can, I make something that pertains to the group, so if it is flowers or animals or whatever. In my area, it seems people mostly like pretty graphic bright quilts, civil war or something muted/low contrast doesn't go over so well. Except with quilters, 30s doesn't go over at all (from what I've seen).
We've had a couple of threads here about the difficulties with raffles in some areas. I have no idea what you might face/not have to deal with in Quebec.
We as quilters are often disappointed with the results of our efforts. My husband's work had a yearly charity basket auction and he came back and reported proudly that my my donated crib quilt fetched $20! I replied that someone then got a very good deal because there was $60 in fabric alone, much less my effort. But it's ok, that was a top for donation -- but that was the last time I donated to their auction!
edit: People here respond well to appliqued borders, even very simple ones like a scrolling vine and simple flowers.