The Pilates Plank is a great exercise, helping align the spine and training your ability to move your legs whilst maintaining a stable core.
Drop into the standard plank position and hold it steady. Now rhythmically and synched with your breath, bend your knee and drop it to just hover above the ground. Continue for half a minute or so.
Here’s my good friend and expert Pilates teacher in Paris, Rita Rimgaile demonstrating the exercise