Get a bunch of people together and let them get creative. It doesn't always lead to useful results. But if you give them the right input, you'll get much better outcomes.
I use a trick with co-workers when we’re trying to decide where to eat for lunch and no one has any ideas. I recommend McDonald’s.
An interesting thing happens. Everyone unanimously agrees that we can’t possibly go to McDonald’s, and better lunch suggestions emerge. Magic!
— Jon Bell in McDonald’s Theory.
If you want to get input from a client for a new website or app, it often works the same way. You often get better input if you start sketching and use it as a starting point for the rest of the project. Those sketches will naturally be rejected (just like the idea of going to McDonald's), but that often clarifies what the project is really about.