I have two places conducive to thinking, and they are as different as can be. First, I think in the car, preferably on a long trip and preferably alone. The automatic process of driving seems to calm my conscious mind and let the deeper thoughts arrange themselves into viable plots and possible characters. I particularly like the drive across Michigan's Upper Peninsula, which is both beautiful and remote, allowing lots of time to work out plot knots. I carry a small recorder and talk to myself, which my kids think is insane. Still, there's a lot I forget once the world reasserts itself, and the recorder helps me keep straight what I untangled.
For shorter jaunts, it's a walk in the woods. When I'm so confused or frustrated by the characters I've created that I can't stand them for a while, I walk. As the distance from the house increases, the calm sets in, and when that happens, the problems solves itself. Once I let go of MAKING it work out, the problem goes away at least nine times out of ten.
I can't speak for others, but these are my best chances for optimal creativity. What they have in common is that I'm alone, I'm away from things that distract me (like the telephone or even the cats), and, possibly most important, I'm NOT WRITING. The two processes are the same and yet different, at least for me. I must think it before I can write it, and only then can I write down what I think.