2019 was excellent. Each day, the puzzle would have you adding new features (new instructions, addressing modes, input/output handling) to the emulator. The early days were fairly simple, but things got progressively more complicated. On one day the puzzle input was code that played a game of Breakout. Your task was to implement an emulated joystick that would play perfectly. The solution was the final score. On another day the puzzle required several virtual machines running in parallel feeding the output of one into the input of the next to get the final answer.

Reddit r/adventofcode is also fun. People post not only solutions in various languages, but visualizations of solutions (e.g., an animation of that game of Breakout). Sometimes people post fun things, like a solution to a puzzle in Apple II BASIC and a video of it running on an actual Apple II.