Squatty Potty The Original Bathroom Toilet Stool, 7 inch and 9 inch, White, (Pack of Two)
I usually gift this two-pack because everyone seems to have multiple bathrooms these days and you can try out both the 7" and the 9", but they have a 7" with a taller converter as well as many more attractive versions now.
Update: I now generally gift the bamboo one that can flip to be either 7” or 9”.
They have a ridiculous explainer video:
It's become really mainstream and there are now travel versions and classy wooden versions, etc.
Note: I have a friend who feel very strongly that the right squatty potty is one that really does tuck all the way under the toilet and just pulls out when you need it. I actually think that the manufacturers agree and the “original” is actually a slimmer, more petite, version than the one I sent around when I found out about it. So if you were an early adopter and you find it sort of “in the way,” it might be time to upgrade. (The bamboo flip one tucks nicely.)