I carry all my stuff in a CamelBak. This one specifically:


It's a 12 L pack, with a 3 L reservoir. I'll usually have a spare tube, multi-tool, tire levers, mini-pump, shock pump, phone, keys, and a couple of energy bars in there. If there's a chance of rain or in case it gets a bit chilly, I'll have a windbreaker/rain jacket stuffed in there as well.

In terms of hydration packs, CamelBak isn't your only option. Plenty of good packs available out there. Dakine (I have an older 18 L pack that I don't use anymore), Osprey, and USWE all have very good offerings.