I use a Camelbak Rogue, which is a backpack style but is pretty minimal. It does have room to add some small items (like an energy bar or two) in addition to the reservoir - but it's not what I would call a full backpack. It's relatively narrow. And yes, it's hotter wearing it than not, but on hot days I want the extra liquid. I usually carry water in my Camelbak and Gatorade in my water bottles.

--- Denise