The 0-99 Chart - the surprisingly powerful standby of early and middle elementary math. Use it to visualize calculations, discover patterns, play games.
I made this clickable one in CodePen, which is an amazing tool that you can use to play and experiment with front-end code. CodePen is a lot like Scratch - you can click the HTML, CSS and JS tabs to "see inside" and you can click "fork" to "remix"! Use the chart and/or go crazy with the code!