How do you motivate yourself to stay productive on an off day?

For me it's important to rest enough 😉 When I'm tired motivation can't help me

I leave my phone at my desk and just go outside for a walk. I find that most times 10 - 20 minutes of walking with my thoughts generally helps re-align my focus. When that doesn't work, I shut off all work-related stuff and do something else for 2 - 3 hours before sitting down at my desk again. It can be reading a book or playing video games or going to the gym. Anything but work.

Take it slow and understand that some bad days does not affect the overall result.

The first question to ask yourself is whether you really need to stay productive if it's an off day. It's okay to take it easy on some days. Rest is productive too.