One of the best exercisers I know to strengthen the core is planks, and I told her this. Well, later she said they were great, but she could not do them on her hands due to another issue with her wrists. That is fine there are many ways to do planks, and they are easy to modify. Recently I asked her how it was going she said good. Knowing she was only doing a basic plank I asked her if she would like me to provide her with variations, and she said yes. This post is what I am going to send to her.
For this plank stay on your elbows and bring your knee towards your elbow, do 8-10 on each side. If you are unable to do 8-10 do as many as you comfortably can, and work your way up. Remember not to lock those joints.
Put one leg up, and slightly push the leg up, heel to the ceiling while maintaining the plank position. Do 8-10 on each side.
I found this great site that has a nice selection of plank variations for both on the elbows, and on the hands. Chose the ones that suite where you are in your fitness journey, when you're ready go back and try some of the other ones. This is a great site because there are not just images, but small videos continually playing.
My favorite is a plank with a row, how 'bout you, do you plank?