We do a spring fair and a Christmas fayre, these are our biggest events in terms of profit, but also organising - takes months of pre-planning and organising and then trying to get volunteers to man the stalls etc is quite frankly a nightmare, but worth the effort as we usually make about £1,500.
Discos are a good idea and fairly easy to organise - book a DJ and charge £3 entry per child, to include a drink and a hotdog. Better in the winter months because then the hall is dark and the disco lights have better effect!
We also hire a local ice rink for 2 hours on a Sunday about twice a year - easy to organise, charge a discounted rate to parents and kids compared to rink prices and easy to run - just turn up and take ticket money at the door.
Bun sales take place throughout the year and these are also easy to organise - we send out letters a week in advance asking parents to bake and bring produce to school on the morning of the sale. We set up tables in the hall for the kids coming out at 3pm and provide paper bags - the general rule is 20p a bun, 5 for a £1. It's literally a bunfight with the kids swarming the tables and we usually sell everything in about 15 minutes flat! Everything we make is profit.
We do class calendars in December, these are a big seller - each child in each class draws a portrait which is then scanned onto a calendar showing the entire class, which has all the school holidays and training days printed on it for the coming year. It's a nice keepsake, seeing your child's self portrait change from wobbly reception drawing to young artist! I'm not sure which company we use though, I imagine there's quite a few available if you Google.
That's all I can think of off the top of my head!