Ask HN: Fun Side Projects?

It's been awhile since I've had any significant amounts of spare time to do a software project just for the pure enjoyment of it, but I'm fortunate enough to have that chance for the next month or so. However, I can't decide what to create! Any ideas or suggestions? I was thinking about (learning) and working with F# and .NET, but am open to any ideas. What are fun projects you'd like to do? Projects done for learning value or projects with a neat end result are both fine.