When I started community I used one of my smaller day packs but it didn't set just right for me. Then I grabbed my sons 6 year old version of this:http://http://www.rei.com/product/778463

Mine has 1 large compartment and 3 smaller outside and 1 smaller inside compartment AND holds a Camelbak 100 oz bladder with no problem. The large compartment also has 4 tie down spots so I can put my clothes and tie them down flat. No shift at all.