From Looney Pyramid Games Wiki

Some Icehouse pieces are hollow inside and can be stacked on top of other pieces. The plastic pyramids available from Looney Labs and some of the stone pyramids available from Crystal Caste are stackable.

A Nest
A Tree

There are two stacks of trios with special names.

Most Icehouse games today use stacking, but many of the older games do not. Original Icehouse sets before 1995 were solid, not stackable. Stackable pieces have been available commercially since 1999 with the introduction of injection molded plastic pieces. Before then, stackable pieces were made from paper, but no games had yet been created that utilize stacks. IceTowers was the first game to exploit stacking, and many games quickly followed, like Volcano and Martian Backgammon. Stacking is also notably used in Precary-Ice, IceCream, and Pylon. See also Stacking.