Here's an oldie-but-goodie plank progression that I've never NOT seen someone have a love-hate relationship with.
On top of the many benefits it offers, two other beauties of this exercise are that it's incredibly scalable, and you can perform it anywhere; you don't need a weight room or crazy equipment to do it. While the video demonstration shows small weight plates being transferred, you can always choose to use books, pencils, kittens, etc. instead.
- spread the feet apart to make it easier, bring them closer together to make it harder
- pretend you have a cup of hot coffee sitting on your talibone: don't let the hips shift or hike up while you're moving the mountain (you'll be tempted to elevate the same side hip as the arm that's doing the moving)
- make sure you actually pick your elbow off the ground.....many people like to cheat by keeping the elbow down and simply rotating their forearm back and forth
- look straight down; the eyes should be more or less directly over the fists
- breathe steadily throughout; glutes engaged; no saggy back; yadda yadda yadda
- use anywhere from 3-6 objects to move, and perform 2-4 transfers of the entire "stack" with each arm. That's one set. Do 2-3 sets, or however many you want to make yourself or one of your clients hate life.
Another way to regress the exercise - for folks who aren't quite ready for the ground, perhaps due to an injury or lack or strength - would be to elevate the arms, which you can do using a bench, couch, etc.