I have heard from quite a few moms that a GREAT way to help your child strengthen his/her core is by using an excercise ball. Holding their legs you help support them while letting them play. (lets not let them realize they are actually working!! : ) )
You can buy excercise balls at pretty much any discount store of your choice.
Now that Toby is older, I realize how important core strength is and has been. So keep up the work!