Transition to Algebra. It starts off in lesson 1 with exploring number tricks the kinds that I first learned about when I read W.W. Sawyer's Mathematician's Delight in 1972. A simple example is shown here in this video produced by Heinemann.
I highly recommend this EDC developed curriculum to any school that's interested in getting all their students to appreciate algebra for the first time ever and scale the algebra barrier. Other algebra texts make learning it insurmountable and the best they can hope for is a fragile understanding. Transition to Algebra makes learning algebra an intuitive and engaging endeavor!