This is an easy addition to a party or fete, and is always a favourite. And it can be good counting practise for younger children, too!
Simply fill a large jar or other see-through container with a collection of appealing sweets. Ask the children to guess the number of sweets and write their guess down, with their name. When everyone has guessed, tip out the sweets and count them. The child who guessed closest wins the lot.
For a fair or fete:
Hopefully somebody will donate the jar and its contents so that you will be able to raise more money with this idea! Have a clipboard for children to write down their estimate, name and contact details, and charge a "guessing fee". You can arrange to deliver the jar to the winning child after the fete.