Originally built as my master's thesis in 2008, Visual Lambda is a graphical environment where lambda terms are manipulated as draggable 2D structures ("Bubble Notation"), and beta-reduction is smoothly animated.
It also includes a small "Lambda Puzzles" challenge, where you try to extract a hidden free variable (a golden coin) by constructing the right term:
https://github.com/bntre/visual-lambda#puzzles
I recently revived and cleaned up the project and published it as an interactive web version: https://bntre.github.io/visual-lambda/
GitHub repo: https://github.com/bntre/visual-lambda
It also includes a small "Lambda Puzzles" challenge, where you try to extract a hidden free variable (a golden coin) by constructing the right term: https://github.com/bntre/visual-lambda#puzzles