Get a couple of graduated cylinders and color code with marker or something the units in liters, oz, and milliliters so your kid can visualize how much liquid is in there and make comparisons.
I bought math and reading books from www.educents.com specifically Life of Fred. They have free shipping for orders over $70 and that brand has math that ranges from k-12 grade levels.
I bought the elementary math series and beginning readers set from them since it was free for purchases over $70. My kids love them even though it boggles my mind that these math books aren't like anything I've had to work with growing up. I would suggest buying additional worksheets to supplement the math drills though. :)