Astronomy Picture of the Day - APDO in short - is a website from NASA posting an astronomy picture including a short explanation on a daily base. Since 1995. I wrote a script that parses their archive into a database so you can search for images by keyword.
Proof of concept with some homemade image processing and machine learning. Hold a paper sudoku in front of your webcam and see the solution projected on the image in realtime. Because who doesn't want to spend their free weekend brushing up their geometry skills.
When you want to go for a drink and instead spend the whole weekend writing scripts in various languages to create the optimal route to all bars in your hometown. An optimal solution for TSP - Travelling Salesman Problem - for several hundred pubs in Ghent.
After solving plenty of cubes myself in a previous life, I decided to jump on the "robots will take our jobs" train and build one that is better at solving cubes than I am. Using 2 old LEGO Mindstorm NXT modules, I managed to build and program one that solves any cube in about 12 seconds.