It totally depends on the kid, but my 5yo plays with those dollar store plastic Army men endlessly. The 3yo has matchbox cars that he carries around the house to race on countertops, boxes, carpets, tables...

Sometimes we set up our little Coleman kids tent in the basement with toys and flashlights thrown in there - that can usually get me through a conference call without crying.

Agree with PP that it’s more about rotation of toys - we find stuff they like and when we truly need quiet time we drag a bin out of the attic that they haven’t seen in a month.

OP, you probably understand these intuitively, but two things:

1) There is no holy grail. At best, there are 5-10 activities that, if rotated, can cover your child the vast majority of the time. The hunt for just one thing that is a guaranteed go-to is probably influenced by streaming TV/iPad, because they are all-in-one entertainment centers with unlimited engaging content. Arguably, there are some open-ended activities that kind of do that, but they are less designed to suck you in, require more work on the "player's" part to stay engaged, etc., so nothing really compares.

An analogue-- especially in the days before kids had so-called screens as a default, and lifestyles were wildly different-- would probably be "running around wild in the woods with a bunch of other kids, exploring, making up elaborate games and building stuff." That's unlimited, compelling entertainment right there, but, you know...

2) To that end, the more solitary (or sibling-only) activities that are best for what you're asking for are the most open-ended ones. So no specific toy, exactly, but like...

-Paint and paper
-Digging in the yard
-Water play
-Banging nails into wood (per PP) or leaving them with a bunch of random wood pieces and rocks in the backyard to do... whatever
-Stuff like that, where they can theoretically create endlessly, and in ways even less constrained than Magnatiles (limited shapes, limited pieces-- not that they aren't also great!)

My 2c.

I agree with all the people suggesting Magnatiles, but want to add that it will help to have a sufficient quantity. 60 pieces does not allow for building much. If you see that your child is interested in them and is capable of building basic walls and structures, I'd recommend having at least 200 or 300 pieces to enable them to build bigger more complicated stuff.

Adding small vehicles, toy animals, and action figures to Magnatiles has worked great for my kids. They like to build barns, zoos, garages, fire stations, schools, apartments, restaurants, and so on and populate them with their other toys.

Huge bowl add some water and little toys, freeze it, add another layer and some more toys and freeze. Do until full. Unmold ice block and give kids a spray bottle of warm water some salt and some tools and stick them in an empty bathtub.

Magnatiles
Sensory bin
Other activities plus a book on tape or kids podcast
Sorting activities

Imagine Ink books (can find in $1 bins at Target)