How to keep productive when waiting for things outside your control?

Sometimes we can’t make progress on our tasks due to situations outside our control. For example: waiting someone to review our code, waiting for a permission approval, a meeting that got pushed back, etc.

Are there ways I can continue to make progress and impact while waiting for these?

I considered having a side project and work on it when I can’t progress in my main task, or getting started on a future task to address it more quickly when I officially start it.