I'm a Camelback user so that solves alot of problems but I carry a spare tube, tire irons and a small Park tool kit in a small under seat bag. I carry my cell phone and keys in a BENTO BAG. If you are unfamiliar with them, google Bento Bag. It's a rectangular shaped bag that sits behind the head/steering tube on the top bar. The large size (which isn't that large) is perfect for phone and keys, or a small digital camera, or fig bars etc.

PS: I stopped carrying CO2 a long time ago - too much hassle . I always carry 2 spare tubes (taken out of the square cardboard boxes and placed in plastic bags with talcum powder), tire irons (just two) the small tool kit in the seat bag and a Road Morph pump by Topeak that I carry in the pump port of the Camelback. I almost always also have some GU or a Balance Bar, some medical supplies such as bandages, and alcohol pads, personal ID, some money - all "just in case." I use a smaller Camelback for the road and a larger one for mountain biking.