If you're not doing the work for someone, then just pick something you
like.  You might make a list of skills you'd like to pick up along the
way, and look for something that matches that skill set.  Without
knowing you or your current skills, your interests, or what you want to
learn... well, it's going to be hard to make any kind of a reasonable
suggestion.

Roedy Green has some suggestions on his website :

http://www.mindprod.com/projects/projects.html