...various lumbar packs in the Mountainsmith line, especially the Tour and Day models. They can be worn as waist packs, with added straps as a conventional backpack,or carried over the shoulder with a sling...
I bought the larger of the two, then went back and bought the strappettes (shoulder straps), as it felt like the load was too high on the belt, making the pack roll backwards, away from me. This has been a good solution for me. The single shoulder strap included with the pack makes for a good short-distance option.
No affiliation, just a happy customer.