What activity do you start a brainstorming session to loosen up the creative muscles?

While in design school, we used to start with turning the brief upside down and try to come up with whacky ideas for the first 15 min- lots of funny ideas and laughs. This was an excellent warm-up for individuals and it also made us more comfortable sharing ideas with the group when we got to the brief at hand.

I always found using a random word generator helpful for me, just seeing a random word and attaching it to something just makes something tick for me.

Discussing our inspiration - sharing other people's work we like. It gets the discussion flowing without having to ideate actively.

I've found on Google; The 100:10:1 method, made by Nick Bentley, and Microscope, made by Lame Mage. I don't know if it helps you, but for me, it works perfectly.))) Discuss ideas with a friend or colleagues, and the discussion will build details from there. We do it once a month at pro essay writing service, it's quite productive.