Leg lifts/kicks.
Take a chair and stand with it to your left, resting your left hand across the top for some support if you wobble. Tighten your stomach muscles and lift your right leg out in front of you waist level if you can. Hold it there and then lower it down. Now lift your right leg towards waist level again but this time out at a right angle, hold it and lower it down. Lastly, kick back in order to stretch a bit and the bring your leg back to standing position.Do a series of these and then switch to standing beside with the chair on your right so you can do the lifts/kicks with your left leg.
Along with the legwork, doing pushups of any sort will strengthen and tighten your core. If you’re after cut looking abs though, you’ll have to burn away fat layer so the muscles below can be show more prominently.