When you don't need the capacity of a full-sized backpack but still need to carry a few bits and pieces, lumbar packs are the ideal answer. The smaller ones are ideal for your keys, wallet, and cell phone (which will be easy to access when you carry it on your hip), and the larger ones work well for water bottles, camera equipment, snacks, maps, and first aid kits.
Nathan VaporStorm Hydration Pack 4519NB
The more generous lumbar bags are popular with off-road runners. When you want to run fast and light with the minimum of gear, putting it into a backpack usually means it'll bounce around. That's unstable and uncomfortable where a smaller hip pouch is perfectly stable. The items inside won't be shaken up too much and there won't be any chafing from a shifting load. The best choices are those than include a water bottle or two- if you get a dual bottle version, try filling one with water and the other with an electrolyte replacement drink. Just make sure to remember which is which or you might surprise yourself!