Products that can be microwaved and "pop" just like popcorn

Popcorn should be considered one of "nature's little miracles" - & a way to make a huge profit out of air.
Yes, you can pop other dried grains/seeds, but don't expect anything quite so bag-filling as maize.

Quinoa, chia, sorghum & amaranth will all pop [in a dry pan, not sure about microwave]

See
https://www.huffingtonpost.co.uk/entry/puffed-grains-popped-corn_n_6107716
or
https://www.bonappetit.com/test-kitchen/how-to/article/get-it-poppin

Popcorn doesn't pop particularly well in the microwave oven. The bag the popcorn is in has a little metal patch to make it work better. You can't just throw popcorn in the microwave and pop it. Hot air poppers and popping corn in hot oil work better.

Popcorn pops because it has a tough shell and a little spot of moisture in the center of the kernel. If you get the kernel hot enough, the water boils and flashes into steam. Steam takes up about 1700 times the volume of the water. That large increase in pressure causes the kernel to "pop." The heat and the steam cook the internal parts of the kernel while fluffing them out.

Other seeds don't have that tough shell. When you heat them, the moisture just evaporates right out – it can never pop because there's no pressure build up.

Popcorn has been bred over the years to make it pop better. Some kinds of corn (maize) naturally had the required tough shell and moisture – popcorn has been popped for hundreds if not thousands of years. Modern packaged popcorn is bred to be more consistent – it pops better and more reliably than the naturally occurring popcorn kernels.

If you want to try other popped seeds, I suggest you try popping corn in a pot with hot oil or in a hot air popper first to see how to do it. Other seeds don't come pre-packaged in the special bag that popcorn comes in for use in the microwave.

Once you can pop popcorn without burning it in a pot or hot air popper, you can try popping some of the seeds mentioned in the links of the other answer(s.)

Some seeds you may want to try:

• Sorghum
• Amaranth
• Quinoa

I have not tried any of those, but they are mentioned in the linked articles.

Puffed wheat and puffed rice (as mentioned in comments to the question) are common breakfast cereals in the United States, but they are not popped like popcorn. Those puffed cereals are made by heating the grains in a sealed chamber, then suddenly releasing the pressure. The old advertisements claimed the cereals were "shot from guns." The heating chambers did rather resemble small cannons, but weren't really guns. The first minute or so of this video shows a small "puffed rice gun" in operation. It does indeed sort of resemble a small cannon.

It is possible that other seeds or grains could be puffed the same way rice and wheat are, but you'd have to have a pressure chamber to heat them in to find out.