Atari 2600, Intellivision and Colecovision will always be the best counsels ever for me. I played a crap load out of them since the 70's as a kid. However, the newer generations of kids just dont have the nostalgia for these classic systems. Here is an idea to perhaps generate some interest and hopefully some nostalgia.
I call it The OG3 Challenge. In reference to the original counsel war.
Contestants bring three different cartridges to play on any system. The convention will provide the systems to play on. Each contestant is randomly chosen to play against another contestant. Each contestant chooses a game that they brought to play. Both contestants play both games three times. Top score from each game receives 3pts, 2nd 2pts and 3rd 1 pt. The player with the most points wins. He/she moves on to play another person. The loser must give up the cart he choose to play and is then given another contestant to play. Lose all three carts then your out of the tournament.. The convention organizers collect all carts and keep them for the prize winner. This procedes until there are four players standing.
The remaining four then play a game of warlords. They also keep any carts they might still have. The winner of warlords keeps all cart collected by the organizer as the grand prize.
Prices are very cheap for most of these games. You could enter this tournament with three carts worth 99 cents each.
Just an idea.