Weight weenie urban trail / broken pavement mixed use rider. Multi-hex wrench, leatherman-type multi tool, fix-a-flat (for exhausting my patience and making a fool out of myself, I'll never be able to use it in an emergency, can't even read the instructions/tiny font), one small water bottle for rides up to 15 miles, a second small bottle on a cross-body sling for rides up to 40 miles, stuffable thermal shirt or rain poncho in under-seat bag, spring-assisted knife (the one with a zytek handle to save weight for longer rides), sometimes a second barely-legal carry of undisclosed type for areas that are remote or have higher than average crime rates. Most recent addition is a second cheap-o lightweight headlight on the bars that has a broader beam than my primary, useful for full-dark conditions or as a backup in case the primary fails, I usually use one on low power after dusk to save battery, then usually one or the other on full power depending on road/trail conditions, and both at the same time when it's really hard to see. Also have been carrying a micro rear light as well for when I forget to charge the primary taillight or my helmet rear light. Phone with full charge in belt holster that also holds an all-weather pen.
I have two tiny bags, one under the seat, and one mounted on the top tube, one water bottle behind the seat mounted on the seat post.