If you are looking for a core builder that's a total body exercise, windmills are one great place to look. Bastardized from the times of old school strength training, these will make you work hard for your abs. They also respect the way the body is made to function as one complete and interconnected system.
If you are new to strength training, you can do those without any weight at all. If you feel confident, start with a small dumbbell or a kettlebell and slowly build your way up over the next workouts.
Jerk or push press a weight to overhead. Lock your elbow and stabilize your shoulder at the shoulder blade. Position your feet wider than hip width. This is your starting position. Take an abdominal breath and hold it. Slowly rotate your shoulder opposite to the raised arm and bend at the waist bringing your body down into the rotation until your elbow is even with your knee. As you rotate down, your opposite hip will shift back (like in a Romanian deadlift). Stay in the bottom position for about two seconds. Brace your abs for the way up. Extend your torso and squeeze your glutes to get up. Push up towards the weight, as if you want to put the weight into an imaginary hole in the ceiling/sky. Once you are up, exhale and get ready for the next repetition. Do 5-8 repetitions on each side. Ideally, you should build your weight up to heavier weights and do 4-5 reps per side, especially if you want to work with kettlebells.
There are variations of windmills, with bending in different directions, with or without rotation, and you can explore them as you get better at these.
You can include this exercise in place of any twist exercise in your program, like a wood chop or a windshield wiper.