Some good water bottles, and plenty of them.

For me the minimum number is 4. My bike has 2 bottle cages, so having 4 bottles means I can use 2 on a long ride, put them in the dishwasher and still have 2 clean ones ready to go for the next ride even if I haven't run the dishwasher yet.

I have a bunch of the Polar lined bottles to keep drinks cooler on hot days. Camelback now makes similar lined bottles.

If you prefer to use an actual Camelback or similar beverage-carrying backpack, get an extra bladder for it.

Also, stock up on spare tubes.