things that are always productive

These might not be the best thing to do at any point, but they're better than doing nothing. And doing them can create a sense of progress that builds on itself.

• Exercise: lifting, running, biking, etc.
• Processing:
  • Editing old writing.
• Going for a walk.
• Sleeping doesn't necessarily create a sense of progress, but it does reset your mood.
• Mental practice: meditation, etc.