Mosswood Connections has a cute idea for a relay race with a STEM twist where kids build towers out of recyclables (you could also use blocks or a combination of materials). They can compete to see who makes the tallest tower, then see whose can be knocked down first.
There are lots of things you can add to this project, from having the kids paint the Olympic rings on posterboard to be the place where they build their tower, to having kids have to run to a pile of materials to find what they need to make their tower. Check out her post for more details.
[Photo: Mosswood Connections.]