This Geometry project takes us about 3 classes (60 minutes each). It sounds easier than it is. I do this with my accelerated class.
Here is the link to the project.
The students have to build a cone or pyramid out of card stock that meet certain requirements. They need to draw a net, a 3D version, and then actually make it. This year I only had 2 kids out of 29 attempt the cone.
They really struggle with this, but it helps them see all the right triangles within the figure. It helps them see where the slant height is.