There are other alternatives to the actual garden produce of course ...
Prettiest plot
Best environmental idea (eg wind generated water pump)
Wines (always popular due to sampling ...
Processed produce, eg jams, chutneys etc
Our association doesnt have events like these, but we have several charities on site, and they have had barbeques etc, so to me it appears that having simple fun should be the objective, so even the less able gardeners feel they can win something.