There's a new meta post about challenge-type questions which may be of interest here. https://math.meta.stackexchange.com/q/35040/29335
It indirectly features the question of whether or not such questions are on-topic anymore. One answer, at the moment, seems to obliquely suggest they are still on-topic, but I believe they are not.
Otherwise it is a pretty broad question but you may wish to weigh in on the topic of puzzle questions.