One of my favorite pieces of workout equipment for training the core is a weighted medicine ball. You'll add just enough weighted resistance to really challenge and fatigue the muscles recruited during exercises like the Russian twist, any variation of crunches on the floor or a stability ball, and will give your lower abs extra work by gripping one between your ankles and feet during leg drops.
Strengthen your core, targeting the lower abs by using a medicine ball to pass from your hands to between your ankles/feet while doing v-up crunches (laying flat on your back, arms extended overhead and legs extended straight, crunching up into a folding ‘v’ motion, passing the ball back and forth).
If you’re working out with someone, try adding some ball throws into the mix. You can either sit next to one another and work your obliques by performing a Russian Twist, passing the ball back and forth for 15-20 reps, then facing the other direction for another 15-20 on the other side, or sitting in a crunch position with your toes facing each other, and throwing the ball back and forth for an extra burn.