C# WPF, what to do?

It is often hard to come up with a 'hobbyist' project.

Ask yourself this:

- Is there some software you wish existed but doesn't or is not available to you? try making that
- Is there a programming technique in which you are less well-versed? find a scenario in which that technique can be used and make that
- Do you have a career goal that leans towards a certain field/genre of software? Consider writing software that will improve your skill in this area and be usable in a CV/Resume/portfolio
- Read some of the questions on this site and see if you can work out a solution (the advantage of this is that by the time you've 'had a bash at it' one of the gurus here will most likely have posted an answer so you can compare your technique with theirs)
- Find a fellow up-and-coming programmer and challenge each other to code more and more difficult feats

Unless you have a requirement for a particular piece of software (which you don't or you wouldn't be asking the question) then the end result of your exercise need not be a usable, marketable application. You could simply set up some code to generate 1000000 numbers and then attempt to refine algorithms to perform simple operations such as searching, sorting, detecting primes, etc etc.. getting well versed in what makes a piece of code more/less efficient will put you in good stead in the future