The step ups would be great, especially if you can move the box to somewhere there is something you can grab onto if needed, like another piece of upright equipment (power rack or similar).
Don't underestimate the steady state stuff for overall health, there are some strong guys who advocate lots of walking (for recovery but also for lower back health).
And lastly, rotate through those other machines. If they are selectorized vs. plate-loaded that's going to be best I would assume for you.
I look at it this way, even if the workouts aren't the ideal you would be able to do with two good wings, the choice between being as active as possible and recovering on the couch is an easy one. I have a bunch of nagging things going on (49 years old) and the one thing my ART guy tells me is "keep moving". Learn to train around things. The worst thing you can do at our age 😄 is just sit still waiting for things to get better.