I've been using the Strong app to save workout templates (in a broad push/pull/leg framework) so I have a list of stuff to do each time, but if I'm bored with an exercise or (more typically) someone's been using the rack for 30 minutes and I need to move on, that's when I'll look for an alternative exercise. Apps like Strong or Fitbod or this new site are great in this scenario - can filter by what you have and will show you how to do it.