I do a routine of 6 exercises. I start with the ball doing Boxer Ball Crunches, 15 to 20 each side. Then I do Hip Extensions on the Ball usually 20 to 25. I put the ball away and get out a foam mat and do 25 power bridges then a plank for 1 minute. Up next is 50 to 75 scissors kicks, counting each time I use the left leg. Lastly is the Transverse plank, 10 to 15 each side. I mix it around a little but almost always start with the ball.
I also do a few other things like regualr crunches, the boat pose and the Catapult occasionally but the above is what I always do. Its quick (10 minutes) and surprisingly effective. I often increase the number of reps when I feel stronger. I hope this helps, visiting a trainer is probably good advise, they may show you a few new ones that are even better.