Option 1 - Seat post rack with panniers (either top mounted or side panniers)

 - You MUST have an aluminium seat post to a seat post rack (a carbon seat post may crack under the load)

 - The following topeak rack is a good choice:  http://www.evanscycles.com/products/topeak/rx-quick-release-beam-rack-ec005992  BUT make sure that you get the right size for your frame!!

 - There are several pannier options which fit with this rack

 

Option 2 - Light rucksack

Alternatively you can use a small rucksack, but bear in mind the following:

 - best to use a rucksack that has "webbing" or a frame to ensure that the rucksack is not against your back (prevents sweaty backs!)

 - select a rucksack that has side pockets and ideally straps so that you can attach items to the outside

 - make sure you have waterproof covers, just in case....

20L should be sufficient for a daysack, something like this one: http://www.evanscycles.com/products/deuter/bike-one-20-ec032261

 

Option 3 - Full pannier rack

ONLY works with touring frames which have eyelets (at the intersect between the chain and rear stays) to accept a full pannier rack

Racing road bikes do not have these, hence only Option 1 & 2 is viable for most people

Option 4 - A Karri|

The "super light" option:   www.getkarri.com