These were used by East German soldiers to carry about 2 days' worth of supplies out on the field. That being said, they make for the perfect book bag (I would know since that's exactly what I used mine for). 
 
They measure about 12"x12"x5" which can carry all your books for school and a small laptop. If you don't want to use it for books, they can easily carry a weekend's worth of clothing, perfect for a short vacation.
 
As seen in the photos, they sport the popular "rain" pattern camo. 
 
These will come with a set of Y-straps, which are adjustable and can be clipped on the pack. Normally, these would also be worn with the equipment belt, but it is not needed to wear the pack.  
 
Most of these will come with bed roll straps, but you can opt out of them if desired. 
 
These are quite water resistant as they are, but some of these will come with a rubber "sock" inside the pack, increasing the water resistance. These will ensure your items will stay dry during those rainy days. Use the dropdown menu if you want one of the packs with a rubberized "sock". 
 
These are used, so please expect stains, fraying, small tears, and/or heavily worn straps. Other than that, these are in useable condition.