another vote for the REI Flash 18. It's a great pack. I use it for day hikes too, and hydration is optional.
I also have a Topeak seatpost rack with a Novara trunk bag. The advantage of the flash 18 is that it weigs nothing and rolls up to nothing so I can commute with the trunk bag and then go grocery shoping on the way home and fill the flash 18. The combo is versatile.