I probably own at least one of everything Sea-to-Summit makes that could support a Camino. Also, I have written often on the forum extensively about the benefits of these products. I continue to strongly support these products. I happen to have the 20-liter day pack mentioned above. Everything the posters, above state about the advantages and uses of the 20-liter day pack are true.
But, I did find something better, in Spain.
Altus, the major Spanish outdoor products manufacturer, makes an excellent, 20-liter siliconized - nylon day pack called the "Giza." You can buy them along the Camino. I think I saw them in St. Jean Pied de Porte in 2014 too.
The
Altus version has better shoulder straps and an easier strap adjustment setup than the S2S version. It also has a second, small zip pocket, where the S2S only has the single main compartment. Here is the direct link to the
Altus product page:
http://www.altus.com.es/Giza_20_1350300_var.aspx
Moreover, the
Altus bag comes in a separate, round, zippered sack, about the size of a hockey puck. The bag has belt loops to mount on a pack strap or a trouser belt. I have one that is attached to the lower part of one of my shoulder straps.
I reach for it there as I enter a town or village so I can shop on the way to my night's lodging. Once I am settled, I use the day bag as my "city bag" for sightseeing or shopping. I also it as my shower bag, to hold everything I need to take into a shower stall at an albergue. If I am using a sleeping bag or liner, it holds my valuables in the foot of the bag. Alternately, stuffed with clothing, it makes a suitable pillow. Finally, it makes an excellent carry-on bag for the flight to and from Europe and the States.
I hope this helps.