But the one thing I'd say is irreplaceable is acting. Watch movies with kids and get them into mimicking accents, and then trying to act like characters. From there, it's easy for them to begin putting on their own "plays" and just having a blast, and the real hard part becomes finding a way to guide them towards improvement (as well as putting up with the quality :P)
For example, one of my kids got into Thomas the Tank Engine. I got her an iPhone game that taught here eye-hand coordination and puzzle solving. Lasted for a week, after which she got into toilet training. There's an app for that (Pepa Bath, pretty cool). You catch my drift...
The short answer is "be a good parent". Sadly for you, that's not a product.
A slightly longer answer is "don't be a controlling parent and provide your children with things to play/interact with".
I think this is about 90% or more of the answer you are requesting. Any products that take credit for more than a very small percentage of "raising creative kids" would just be marketing hyperbole.
People have theories about what makes kids creative, but without metrics, it's just speculation.
Id also agree with lego - but the modern lego kits all seem to have custom components which you will struggle to reuse in yoru own creations. A comparason being the 80's lego my son has vs his current superheroes - he's a lot more imaginative with the 80's lego
But what she realized in few months, that our little one plays with duplo in the same way she plays with her plushies, what in my mind as well was not really utilizing the strengths of duplo.
So then she bought her some more discrete pieces. Weird legs with suction cups. Butterfly wings. Head of the giraffe. Weird eyes. Spider legs that can jump.
And then my daughter built an awesome frankenstainian monster utilizing all of the body-parts, she called it Chomper and it was her best friend for a month :-)