Some necessary ingrediences in the end of the HoTT book. (In an old version, it was around p.44 and at the very end.)
A year ago I asked a relevant related question here
Think about what would be a good order of introduction (using WAT) (see Outline). E.g. adding 5+3 would be among the very first things to do. For educational purposes, talking about $\to, \times$ before more intricate concepts is also a good idea. As I thought about before, going bottom (Hands on stuff) to top (abstract minimalistic stuff) with the typey/computational/constructive stuff should be a good idea. Then, for sets and cats, it's the other other way around.
\prod
And I mean “Stuff that eventually needs to be available to do basic computation”, otherwise the answer is “everything”.
notions to play around with
$\to$
$\lambda.$
${\mathbb N}$
$0, 1, 2, 3, 4, 5, 6, 7, 8, 9$
$+, *$