The forest fire cellular automata was a lot of fun to work on and watch. It has now been updated thanks to a welcome pull request which adds a Speed slider to the UI.
I extended it further to add some simple procedurally generated mountains and lochs (lakes). One challenge here was to keep the time down (it takes a lot of recursion). A quick way to get some speed was to reduce the resolution so each cell is 3 pixels square rather than 1.
The interface isn't as phone friendly as it was but is still usable and runs fine and at a decent speed. This thing will happily keep a core of your CPU burning so be sure to close the tab if you are on low battery!
Thinking ahead I would like to make the mountains less round and add an isometric view. Feel free to beat me to it with a pull request. Enjoy!