Formal:
* Idea generation
* Concept development
* Creative thinking session
* Innovation workshop
* Problem-solving session
* Mind mapping
* Lateral thinking
Informal:
* Spitballing
* Thinking outside the box
* Coming up with ideas
* Generating solutions
* Putting our heads together
* Having a brainwave
* Throwing ideas around
The best alternative for you will depend on the specific context and tone you want to convey.