I have a project to measure the diameter of the sun using card and a ruler.

LEVEL: Intermediate / Secondary

This is a great science activity which is really clearly explained in the University of Berkeley site. To get to the relevant page, go to Finding the Size of the Sun and Moon.

Hot tip: You can trust this information because it's put out by a reputable organisation, University of California, Berkeley. You can tell this because there is a .edu in the web address, meaning it's from an American university site.