This is fun! Here’s RPN in my language (as yet unreleased, but very similar to D): https://gist.github.com/FeepingCreature/2fcc82cecdf27b4301d3feed06fcc5bf
Forths are nice. They are similar to Lisp in that they’re very powerful in relation to how little effort they take to get going. If you just need some runtime control flow logic somewhere in an unrelated piece of code, putting a Forth in is probably the best bang-for-buck.
Personally, to learn a new language, I usually write one of Mandelbrot, a Fractal Flame implementation, or a raytracer. Because pretty pictures~