I always include planks, (usually dynamic) in some form or another in my classes and training sessions - so good for building core and upper body strength but can also work every muscle group including glutes, hamstrings and back.

Shown here from top left:

1. Static straight arm plank - hold for as long as possible, aim to increase time every other day.
2.Straight arm plank with alternate leg raises
3.Wall plank - Don’t be deceived by photos! I could only hold that wall plank for a couple of seconds - it’s waaay tougher than it looks. Let me know how you get on with this one, I'd love to know!
4. Side plank - Hold for as long as possible on each side
5. Straight arm plank with alternate shoulder taps
6. Renegade Row or straight arm plank with dumbell rows - try and focus on stable form without weights before progressing to dumbells and increasing weight.

Choose one of these, give it a try every day for a week and let me know how you get on!