Here's a GREAT mathematically based Physics / Earth Science activity! I use it in Honors Physics (Juniors) and get great results; within 10% which ain't bad for government work, eh? For lower age groups, the math needed can be cumbersome, if not downright ugly and incomprehensible.
I can't draw diagrams well online, just doesn't work for me, no talent and stubby non-cooperative fingers... so you'll have to download the instructions and diagrams HERE.
It's a simple method with kinda simple data manipulation afterwards. You watch the sun actually SET TWICE. From a prone position then a standing position. The time required to set and your height (distance between your eye positions, actually) and some cute math will yield the radius of the earth. COOL BEANS!