Good backpack for carrying groceries So, I need to be able to carry 3-4 days worth of groceries for one person, about a mile from the store to home. 
I'm not too keen on the idea of panniers or anything else that mounts to the frame. I don't want any extra junk burdening me during the 99% of rides that aren't grocery runs. 
So, I've been wearing a backpack but its a little small and not well suited to carrying groceries. 
Does anybody have a suggestion that will work in my use case?
I don't want any extra junk burdening me during the 99% of rides that aren't grocery runs.


I use a Bontrager pannier (or two) like this one: http://bontrager.com/model/08102

The rack on my bike is permanent, but you can clip the bag onto onto the rack, or detach it again, in a second or two: so put it on the bike only when you're going to the store. I also take the bag into the store so I can fill it there, then bring it out already full and attach it to the bike and cycle off.