I like to come up with theories about the universe, inventions and our planet. I then research to see if others have thought the same things, if not I elaborate on my theories to make them more likely to be true.
I recommend keeping a note pad and pen next too your bed, you will get some great ideas when just about to drift off. And keep a pen and paper on your side at all times, just in case you have an idea. I will not be giving examples, as I guard my ideas closely.