You can get some pretty nice sized trunk racks and put them on a rack that attaches to the seatpost. Check out Nashbar. Just remember, you really need to keep it light. Combine that with a handlebar bag and perhaps a camelback with a pouch and you can ultralight tour, particularly if you are staying in hotels. This would give you room for a couple of days of clothes, snacks, camera, etc.

Keep the load light.