When mine were little, they absolutely loved going on buses and talking about things we could see out of the window - nice cafe at the end of the line, bus back, hey presto, half a day gone! (And we lived a mile from the nearest bus stop.)
Puddly walks in wellies and rain suit.
Sheet over the kitchen table for a tent.
Get them to line up and name all the soft toys.
We had an indoor ball that made lots of different noises randomly, and just kept rolling it back and forward with them, great fun. (NB it only made a noise if you rolled it - I couldn't stand noisy toys they just switched on and made a racket with.)
I like the idea of stacking lots of kitchen boxes, I never thought of that for mine! But we did have stacking toys that they would patiently watch me build over and over so they could knock it down ...
Slightly older:
Lining up their toys and the chairs for a "bus game", they are the driver (with steering wheel, I think they used a plastic plate).
"School" and they are the teacher.
Kitchen shop with a fold-up table, play-money and all your tins.
Playing "cafe" with child's tea-set (this one never lasted very long, iirc).
Dressing-up with a bag of grown-up clothes (mostly blouses, sweaters, socks, shawls and hats). You can take a photo of each "outfit" if you like, keeps them going.
Reading a book with them.
Baking a cake, or build-your-own pizza.
Play-doh or similar (we always kept it well away from any carpets), especially with the little machines you can make it into shapes with, they love doing stuff.
Jigsaws.
Visiting friends with kids similar ages. And doing jigsaws...