I think an excellent contest would be one that implements the
Condorcet Voting method ( explained for example, at
http://electionmethods.org/ ). Since it's a more fair voting system that improves immensely on our current voting system. This would be a worthwhile and very real life project IMHO.
I'm sure this could be done with PHP and MySQL although it will probably get far more complicated with a variable number of candidates (adding more to the programming fun and learning!). Anyone else think this is a good idea?