Stationary bike, rowing machine, or simply focused on strength training and lifting, doing seated exercises, of course, if you have access to a Pilates reformer or doing Pilates mat exercises, then do that.

Honestly, let the injury heal, your feet are your foundation, if you mess up your feet, you are pretty much screwed, it's better to take the time to let the stress fracture heal, rather then to incur further injuries like fallen arches/flat feet, plantar fasciitis, or other tendinopathies /deformities of the foot.

 

Rationally logically, if you've worked out to the point where you are incurring stress fractures, then you really do need to take a rest. Also, you might want to think about supplementing with calcium and vitamin D (A bone health supplement should contain calcium, magnesium, phosphorus, and vitamin D) it will help your bones heal faster and will help strengthen them for the future.