I've got a small seat bag with an extra tube, patch kit, etc. For most rides I just have that and stick my i.d., phone, and a snack of some sort in my jersey pockets. But I also have a rack on my bike so for longer rides I attach a trunk bag to hold more food, a jacket, lock, or anything else I need. For commuting, I use a larger pannier to hold whatever I need for the day.

Sarah