I have found that, for simplicity & ruggedness, tactical tailor makes good bags. I have a daypack that I've used for several years, and not in the most hospitable environments. The one I have has 4 pockets: an outside pocket, sorta like a map pocket; a middle pocket, whic is designed to put in stripped MRE's about 3 days worth (easily); and the main compartment, that can hold enough gear for several days. I have an older version of the one on this page: http://www.tacticaltailor.com/index.asp?PageAction=VIEWPROD&ProdID=294
Mine has side compression straps, where you can also attach a couple of bags. there is an inner sleeve for a hydration system as well. It isnt waterproof, but, it is definitely rugged.