Maybe you've heard of problem solving techniques such as "attributes listing," and "concept combination." More creative thinking doesn't come from just knowing these techniques, though. You have to use them until they become a part of your habitual thinking process.
Imagine you want to invent a new bicycle. If you've trained your mind in "assumption challenging," you'll automatically begin to ask things like, "Are wheels necessary?" "Does it have to go outside?" What if the "bike" was indoors, and pedaling it ran a video screen? You could "steer" through endless different scenes.
You won't always have great ideas, but you'll have enough ideas to make it more likely that you'll find a useful one. This "spontaneous" creativity will be because of your brain training exercise. Why not start developing those habits of creative thinking?