One of my students came up with - what I thought - is a good idea for an Exploration: finding the volume of a donut. Or, more precisely, deriving a general formula for the volume of a torus. The torus (donut) can be interpreted as a solid of revolution where the solid is created by revolving a circle of radius r around a line such that the center of the circle is a distance R from the line (axis of revolution).

Here is an image of a torus graphed in 3D parametric mode on a TI-Nspire CX (using the software rather than the handheld device).