You, for sure, have heard about brainstorming! Starting from a question, a statement and trying to get as many ideas/solutions as possible. This technique can be used individually as well as in groups (5-10 participants).
Especially when in groups, it's important to apply some basic rules and to ensure before starting that everyone is aware of them and that everyone is on the same page regarding the topic.
The Number 1 rule is: No Criticism! All the ideas are good and crazy ones are welcomed. To avoid negative thoughts, people shouldn't use the word "But" and prefer "Yes and..." when discussing.
A brainstorming session should be directed by a facilitator who will set and give a timing and ensure the group sticks to it. He/She will help in case people are stuck, keep the focus and will enable everyone to participate. Don't forget that during a brainstorming "stealing is ok": you can build on others' ideas!
Don't get stuck with one solution, Brainstorming is more about quantity than quality and a method to Open everyone's mind can be used before starting.
Post-its are often the best solution for brainstormings, you can write but also draw on it and use one of the following techniques to help the group being more creative.
The "upgraded" 6 thinking hats: write down 6 ways of thinking on papers, it can be "pessimist", "futurist", "kid", "elderly", "poor", "athletic"... or anything that fits with your project. Then every member of the team picks a paper and will have to think with this state of mind during the brainstorming. It can be hard to put yourself in a different state of mind, but the goal is just to give another point of view to maybe widen ideas and it shouldn't limit you in finding any kind of solutions.
Release a constraint: it can be a good way to come up with crazy ideas. It can be small constraints as well as much important ones like gravity, power need, handhold...
Look for the worst ideas: What would be the worst that could happen? Which future you definitely don't want to see? What wouldn't solve the problem at all? And then think about what could be the solutions to avoid these "disaster situations".
Exaggerate the problem: pick one property of your future product/service and exaggerate it. For example, instead of making the waiting time of a service "agreeable", try to brainstorm with "entertaining like Disney World".
You can also check the Braindrawing or the Brainwriting methods. All will be useful in Co-creative sessions.