Right now, when I need to tote something, I just use a backpack but I'd like to not have it on me but rather something attached to the bike.  The kicker is, I need it easily removable so I can tote it into and out of work with me.  I'd like to make minimum to no mods to my bike but I do not even know where or for what to start looking.  I want to be able to carry some raingear and a lunch sack and a bit of paper work is all so I don't really need anything that big.  Any suggestions.