We had a week long of daily security related challenges ranging from easy to hard for everyone to participate. From simple sql injection and password encoding to buffer overflows. Some were coding exercises, others were open ended questions, googling encouraged. Every challenge gave a number of points and at the end of the week there would be some prices for the winners.
A lot of people participated and personally I learned a lot and had fun doing it.
May require some effort to set up, but then again these may already exist online.