Injured foot -- what are your favorite no-feet-involved strengthening exercises?

Hi runnit! I injured my foot a week ago while running my first marathon. It's healing, but I'm not sure how long it'll be before I can run again. I want to spend some of the time I would be running doing some core and upper body strengthening stuff (like sit-ups, that kind of stuff; I don't have weights or access to a gym). What are your favorite exercises to do that don't put too much weight or impact on the feet? I can stand on both feet, but definitely don't want to do any jumping or put too much pressure on my injured foot. Links to resources or videos would be appreciated as well. Thanks!!