Fish tank starts out pretty sad. One egg, empty space, nothing moving yet. Come back later and suddenly you've got baby fish everywhere, random decorations you don't remember placing, and a setup you've quietly become protective of. Full version unlocks more fish, more tank clutter, more stuff to mess with when you should probably be doing something else.
Doesn't ask for much. Check in, hatch a few things, move a plant two pixels to the left, leave. Offline growth keeps things ticking over, so there's usually some small surprise waiting when you reopen it. Hard to explain why that works. It just does.