One of those expandable seatpacks is good. Mine holds waterproof, tyre levers, multitool and a couple of energy bars. Phone travels in pocket. Money and cashcard get wrapped in a plastic bag and put in seatpack. It also has a small zip pocket on the bottom that's just the right size for a spare tube.
For longer trips or in uncertain weather I put a camelbak insulated pouch and a whole cycling wardrobe in a small rucksac!