One of the ways to maintain focus in a "club" of this nature, would be to have a challenge that is based on a theme ... Gamedev.net (IIRC) used to run a yearly competition that was based on the four elements (Fire, Water, Earth, Air) as an example.
As a general rule, any project that would take more than 6 weeks is probably far to complicated to complete for someone who is new to development.
Tips:
- Don't code from scratch, use an opensource game engine or take advantage of frameworks like Open Scene Graph or OpenSG
- Look at simple things people enjoy doing (puzzles, board games, card games) to draw inspiration for what you want to make; starting with complicated ideas will only lead to projects that are difficult to complete.
- Focus on the core gameplay element, goldplating requirements is not a good way to start