Sit ups or planks if you have back issues are good for the core. I have also done some exercises with a dumbbell where I hold it out and rotate my body while keeping my head facing forward. Most core exercises also seem to help with posture. If you do a web search for longer drive workouts (or something like that) there are a lot of articles about workouts for golf that strengthen the core.
I also like using resistance bands when working on my core.