I'm in a similar situation and mindset.
Things like the problems in books and Project Euler on my afternoon days off or at night sometimes just don't cut it in terms of grabbing my interest. So, I started saving links to coding puzzles for just this purpose. Here is my short list so far:
(Sorry, I can't post more than 2 links without 10 reputation.)
Reddit Puzzle: http://blog.reddit.com/2010/08/reddit-is-hiring.html
I have not looked into this yet, but UVA Online Judge has problems and I have heard as a recommended source for something like this: http://uva.onlinejudge.org/
I started the Facebook Puzzles, they are interesting and enjoyable.
My one other thought was to take old Atari games, learn some SDL (or maybe not) along the way, and re-write them in C as a learning experience. They were originally written in assembly I believe, had limited memory, limited graphics, and thus writing them in C is fairly high level I would think and not be too large for some.
You could try something like Top Coder too. A little competition is a good way to learn real quick.