A good kettlebell core exercise would be all of them. heh but yeah but in all seriousness i would second the windmills. I do russian twists on the ground sometimes with a 1.5 pood or 2 pood kettlebell. with a heavier weight, these take very minimal reps to feel your whole midsection tighten up.
i think one of the best things is to just focus on your stomach while you swing. when i didn't understand the whole "breathing behind the shield" concept, i used to open up way too much between my lower ribs and pelvis. then one day i was walking through a local park and saw a guy pulling a ton of kettlebells out of his car. he was Dave Whitley, a pretty big name guy on the RKC levels. he evaluated my swing and corrected it in a flash. I stood there and started swinging, and then at the top of each swing (right when it was at nipple level or a bit below), he would push the kettlebell down. at first this was a dramatic change and it loaded my posterior muscles so much harder at the bottom of the swing. after doing several of these he told me to simulate the same feeling myself, meaning throw the kettbell down hard at the top of each swing. that's when your abs and obliques kick in hard. in order to stop that upward momentum, you have to keep your shield tight, just like you're going to take a punch. if you're abs are relaxed and weak, then you won't have the ability to throw the kettlebell down hard. try to get a friend or somebody to push the kettlebell down during your swing. have them do it just a bit at first and then gradually throw it down harder for you. once this feeling is burned into your brain, throw it down yourself.
other core stuff that is kinda fun is the gladiator press and its variations. basically you get into a yoga side plank but hold a kettlebell in the top hand. then you press the kb or dip your hip or whatever you want to have fun doing