I use this 10’ Stainless Steel Cable with a small loop in each end, and a small padlock to secure  my stuff.  I loop it through my saddlebags, helmet and backpack/camelback and lock it to the bike when I run into a store or restaurant.  Packing light is important.  This cable is light, compact and takes up very little room.

This won’t stop someone who is determined and equipped but it will protect you from someone grabbing something off your bike and running away with it.