Here are some of my fun side projects.

AAL An imperative language with a interactive web-based runtime for visualizing algorithms on arrays/lists (e.g., sorting).
MCTS A MTCS+UCB agent for checkers in C++.
Mal A complete implementation of the Mal dialect of Lisp in a variant of C.
Simp Solver-aided IMP: a small imperative language with automatic verification via compilation to Z3.
gdtypes Parser and typechecker for GDScript.
hakan A functional programming language with algebraic datatypes and typeclasses. Includes an interpreter and JavaScript backend.
monads Monads from adjunctions in Coq.
puddi A 3D graphics engine written in C++/OpenGL and tailored for real-time, interactive applications. Not maintained recently, but it has a somewhat interesting scene graph system.
puddi.js An object-oriented 2D graphics library for the browser.
WebGrumpy An interactive web interface for visualizing token streams, ASTs, and code produced by the Grumpy compiler.