When I was packing my rucksack today to go to work, I thought that’s time to use it for a few rucksack curls as a barbell substitute to train the biceps.
I stand straight with slightly straddled and bended legs and I hold the rucksack on the top handle with my right hand. Then I start lifting it upwards while bending my right arm until my hand is more or less in one line with the right shoulder. After a few repetitions I change to the left arm. Of course the back always stays straight. What you put into the rucksack and how heavy it should be it’s up to you.