If you'd like some extra room on your bike to carry things on the back, you can elongate your bike with an old bike's frame and some electrical tubing.
The project is remarkably cheap, costing only about $US20 (or less, if you have some of the stuff lying around already). The instructions do include welding, and while some welding skills would be ideal, the author also offers some suggestions to those that would prefer to avoid it. The original bike isn't altered in any irreversible way, either—you're essentially just adding a second bike frame to the back of the original—so you can always put your bike back to the way it was, too. Hit the link for the full instructions, and if you'd prefer a full-on bike trailer, we've mentioned ways to do that too.
Xtends - Convert Your Bike Into a Long Cargo Bike [Instructables]