Pump, patch kit, tire lever(s), spare tube, multi tool*, and a seat wedge or frame bag to put that stuff into- that's the bare bones for anyone riding. Then next up would be based upon your riding environment- hydration, lighting, lock(s). Everything else will be personal preference derived through trial and error.

*I use the Park MT-1. Anything I can't do with this with my limited wrenching skills is best left to the LBS.

**Also would say either bus/cab $ or cell phone ranks up there as well...