I carry 3 water bottles: 2 with water and one with Ultra Fuel solution. This is for longer rides though. Shorter rides, I still carry 3 bottles, all water.
In addition, I carry the following:
- Spare tube
- multitool
- patch kit
- cell phone
- piece of duct tape (it can be used to boot patch a tire if the tire casing gets damaged badly in a flay, might save me from having to use the cell phone for that "call of shame")
- Ride nibbles, generally Clif Bars, Ghu and gels give me a slam and sugar crash syndrome. An alternative sport nibble is Jelly Belly Sport Beans (A jelly bean w/ electrolyte supplementation! COOL!)
- Raingear
- Digital Camera
- pump
I use a trunk bag though on the rear rack. Almost anal preparedness, but it's nice to have if you need it an d a nightmare if you need it and don't have it!