Keeping fit at work?

I recently got a summer job stacking shelves in a supermarket, one of my goals for this summer is to get in good shape before I go back to university and I’ve decided that there is a lot of opportunity during my longer shifts to try and work on my body, would there be a way I can make the most of lifting heavy boxes and stuff to try and tone my torso? I get this is probably weird but if anyone has any ideas let me know