Standard flapjack is 2:3:4:6 syrup:butter:sugar:oats (my tray bake uses 2oz as Base unit so 4oz, 6oz, 8oz, 12oz). Melt together the syrup and butter, then mix into the sugar and oats. Press into tray lined with good baking parchment or silicon paper, and bake at 180 until caramel is bubbling and dark. Leave to cool slightly but cut before it sets.
It's fairly flexible for additions so long as they end up well coated in the sugar/butter/syrup mix. The Crackjack version had 2oz ish of sultanas and 4oz ish (? a bag, you know) of chocolate chips folded in.
Don't make it until I've checked I've got the sugar and butter ratio the right way round though 
(am MNing in bed and am about 90% sure).
Brownie is Hummingbird Classic with quantities doubled for my tray. Substitute out the 130g flour for about 175g cornflour to make it gluten free (but check other ingredients particularly chocolate very carefully).
Firework cake is normal 2:2:2:1 flour:butter:sugar:egg sponge with (300g icing sugar:100g butter:40g cocoa:40ml milk) chocolate buttercream with additional food colouring and a fuckton of sprinkles. Children hoover it.
Ginger cake is David Herbert at the bottom of this page. I don't bother with the additional ginger syrup step: it's a very moist cake already.
Rice crispie cake is two bags of toffees melted with one bag of normal marshmallows and a knob of butter, then fold in enough rice crispies so they're all only just covered. Lined tray, or use lightly greased gloves to shape into dinosaur/castle/boat or into single cake cases. Decorate with sprinkles/chocolate chips quickly before it sets.