For times when you don't need to carry a lot of stuff, something like a Carradice Barley is a nice option. About the same carrying capacity as a trunk bag, but without the need for a rack.
I've got a Barley that works great for spring or fall rides where the weather might be changing a lot. Plenty of room for a rain jacket, vest, arm warmers, tights, some gloves, etc.
I suppose a disadvantage might be that it's more difficult to use with smaller frames, though..
Steve in Peoria