Ask HN: What type of challenges do you like to code for fun?

We're building HackerRank.com with the core part being fun challenges with a social component around it (add your school/company network). We just put up a launch page last week with an interesting game and we had over 500 bots with 2.1M games being played till now. Completely surprised that people from Facebook, Uber and Hulu had so much time and interest in coding bots.

Gearing up for our beta launch, we'd love to hear from you on what are some of the kinds of challenges you like to code for fun? (game bots, scraping stuff, heavy-math problems, quick stuff like 4clojure.com, etc) Your feedback will greatly help us in building. Feel free to ping me off list. Appreciate your time.