The obvious things to have at your school carnival fundraiser are games and food. Things like ring tosses and knocking over pins with bean bags to win prizes are great little money makers, and if you can get the prizes donated by local merchants in exchange for on-site advertising, that will mean all the money you make from the games goes directly to the school. Likewise, if parents will donate their time for preparing food and the ingredients can be mostly donated, then all the food sales will be profit, which is a fantastic result for a school carnival fundraiser.
Crafts are another great way to make money at a school carnival fundraiser. If you send a note home to the parents several months before the carnival, you can find out who is willing and able to help make knitted items or other crafts. Even the kids can help out by painting their own art, which can be sold at the school carnival fundraiser.
Lastly, you can always have booths with special school carnival fundraisers like chocolate, or a group of people mingling through the crowd with scratch cards. Scratch cards are a quick and easy way to get that little extra bit out of your school carnival fundraiser, and with profits up to 90%, it's hard to turn down a school carnival fundraiser opportunity like that.