I am with Bike Writer, I like to carry my "stuff" on even short rides. First aid kit, snacks, small suncreen, house key, mini-tool, kleenex (lots o kleenex!), lip balm, etc...
I have a small trunk bag with expandable pannier type bags on each side that unfold if I need them. I love this set up! This way I have more space to stash an extra light-weight jacket or spare gloves or if I stop to pick up a few items at the store. It also has a zippered expansion panel so it will hold more volume. I used this when I commuted to work this summer with a small mesh back pack and it was all I needed.
If I remember right, I got it at REI earlier this summer. It's a Topeak brand bag.
"Don't go too fast, but I go pretty far"