When I was 8 or 9 or so, my parents decided I wasn't getting enough stimulation in the classroom. They probably decided this because I was almost always bored and never had homework. My school didn't have much of a gifted program, so my parents hit on the next best thing: Odyssey of the Mind. I credit that program, and Destination ImagiNation, which spun out of it later on, for the bulk of my creativity.
Both programs follow the same model: Get a group of kids together, get them to choose a "challenge" from a list, and then have them solve that challenge before a certain date, when they'll present their work and maybe move to a higher level of competition. Each challenge is essentially a short play based around a statement like "Music can tell a story" or "Travel is a constant theme in fiction". Challenges come with a set of requirements that vary each year (don't set it in your country, don't use verbal communication, build something that moves across the stage, write a song) and requirements that never change (one challenge always involves building a vehicle, another always involves building a structure that'll hold weight, all challenges have a budget). "Solutions", as they're called, get scored on creativity, and there are few enough constraints on the teams that they can do pretty much anything they want to. There's also a secondary challenge that's much more spur-of-the-moment, and again, creativity will get you extra points.
The emphasis on creativity is paired with an emphasis on independence. Adults aren't allowed to contribute to the solution, though they're allowed to teach and supervise so the kids don't, say, cut themselves with a saw or spill glue on the carpet. Everything else, from choosing a setting to designing a set, from making costumes to writing a play, has to be done by the team, and the team's aware that they'll be scored on creativity in all areas.
I did OM and DI at least five times (memories of elementary school are a tad fuzzy). I came away with the ability to brainstorm on the fly, which has helped me solve plotting problems more times than I can count; the mantra of, "If they don't say I can't do something, I can"; and a tendency to combine the above so that I'm almost, but not quite, breaking the rules. For instance, nobody ever said we couldn't make a set out of empty boxes….
I highly recommend reading up on these programs, their philosophy, and the challenges. The "instant" or "short term" challenges are especially good for teaching brainstorming skills, if you can get your hands on some. Try them with friends, or your family, or your writing group, and you'll see improvement pretty quickly. (Or I suspect you will. No promises.)
And definitely get your kids involved in one of these programs, if you can. They'll probably hate you at the time, but thank you later. The lowest competition level is kindergarten. The highest level is university. If you don't have kids, volunteer! Teams need managers. Competitions need appraisers, organizers, and people to compile scores or set up sound systems. There are probably more jobs as well.
This is a completely unpaid advertisement, by the way. It's not what I was intending to focus on at all. I was going to focus on the "me being creative" aspects, but … those weren't really as interesting as I thought, so there you go. Plugging. I do this sometimes.