I don't have time to waste this weekend, sadly, but if you do, there's a challenging little puzzler called Putty Puzzler, found over on the interestingly named 'Coke and Code.' It's putty. It's a puzzle. It's kinda hard. I spent a little bit of time with it and was pleasantly challenged — I'll come back for more after I've got a little time to waste.
Putty Puzzler [Coke and Code via Rock, Paper, Shotgun]