Doodle wildly and super roughly?
Listen to inspiring music
try to do music inspired super fast sketches, try to finish it till the song is done (I often do this rofl)
Exaggurate anatomy and poses

Doddles and action poses sound like they would be helpful!
Maybe anatomy studies too?

Could try! c:

Do some gesture/life drawing perhaps? In class we did 30-1-2-5-10 etc. Exercises where we'd pull up a pose, and then give ourselves 30 seconds to try and sketch it before the next one. Then after a few, one minute. Then 2, then five... Etc. So on so forth. It really forces you to loosen up because you don't have time to micromanage.

Don't be surprised if you get like... Three lines in thirty seconds. It's like stretching, you're going to start out stiff and slow if you haven't done it for a while.

There are sites like quickposes to help with this sort of exercise if you want to just jump in c:

Draw stuff that makes you happy, also drawing circles before starting an actual drawing loosens your wrist up

I concur. gesture drawings with time litmit force you to loosen up.
Though I shamefully admit I hate doing those and switch to larger things too soon xD

Or if gesture drawings of people aren't your cup of tea, rough quick sketches of things you like to draw also help