Anymore for all-day rides I’m always taking my camelbak with about 1.5 liters of water with a hydration tab. 2 packs of GU, Trail Mix, my Crankbros 19 tool multi-tool, Genuine Innovations Plug kit, some sort of inflation device (whether it be co2, pump or both), a spare Derailleur hanger, Extra Chain Master Link, and cellphone.
If I’m racing ill just use a water bottle with a hydration tab, 2 packs of GU, Ill put my multi tool in the back of my jersey on one side and put the Plug Kit and CO2 inflator on the other side. Seems to keep the weight way down and just be enough for when I get into tight situations. Also the center of gravity is a lot lower not having the extra weight from the camelbak tied on my back above the waist (maneuverability is a lot better).