You could always use a Deuter bag if you don't want panniers. THe bag has dual access to the top loader. YOu have top access and bottom access. Also the back of the bag is fully vented in a suspended (right term?) wire mesh frame which if I reclal right is about 1" out from the back of your pack so you get the max airflow on your back to keep cool. It's like you don't have a pack on the back in a way while having a pack on the back if you get what I mean. Bags come in small to large sizes (~50L). I'm saving my coinage for a mid-sized 32L bag for carrying stuff without a pannier/rear. 32L is about middle of the road with good space yet not really giving a 50L look to it. I've tried that pac on before and it works well. Only problem is I wear a duty belt with beltline kit. The 32L mens is a little lower then the hip. The ladies version is a little higher up the hip allowing beltline kit access while mobile.

http://www.deuterusa.com/products/hiking.php

Zero_Enigma