I spent a lot of time learning new languages and filling in my missing computer-science education. You can do a lot in 3 months if you have no distractions! I was probably there 60 - 70 hours a week and have never managed to hit such a sweet spot of motivation, inspiration and productivity before or since. My PhD was a colossal waste of time in comparison. Actually it made me totally re-evaluate my ideas about education in general.

A random selection from memory:

  * Emulator and debugger for a particular embedded hardware (MSP430)
  * Website that scraped Rosetta Code and displayed it in a less-annoying form
  * Genetic algorithm for graph problems
  * Nand2Tetris course
  * Two weeks trying and mostly failing to learn Haskell (took me another year to really 'get' it)
  * All sorts of other fun/dumb stuff. Boggle solver! Endless project Euler problems! Super Meat Boy!