I went to an offseason strength and flexibility class this winter where a lot of the floor work was yoga-adjacent. Various types of planks. One I thought was really excellent was to get on all fours and then raise the opposite hand and foot straight out. The twist is to put a squishy ball under one of the support points (hand or knee) while you do this. Move the squishy ball to a new hand or knee and repeat until you've done all four.