You could probably fit a bag inside, a bivy sack would very easily fit inside, but there are straps on the bottom of the bag that would allow you to attach a sleeping bag on the bottom of the bag. There are also waist straps, very adjustable, and a chest strap (with elastic for comfort). Both the chest and waist straps are removable. I am a pretty big guy, and my only complaint is that the bag almost seems too small for my frame. I love it, but am used to having something larger on my back in the way of a backpack. The waist straps and chest straps fit me with room to spare. I have removed both for everyday carry of the bag, but they would come in handy for a day hike. In addition, there is an Y shaped compression strap on top of the bag that would allow you to add a climbing rope, blanket or poncho to the bag as well. While the Condor only holds 1520 cubic inches (25L), it holds a lot more stuff than I would have thought. I haven't overnighted with it yet, but for an every daypack, an overnight, or even a one person B.O.B. it would do nicely.