I think fun in learning comes mostly from the "intuition reward". If you understand and _feel_ some abstract concept in an intuitive manner, you will feel happy and you will probably want more of that. So, for teachers finding ways to teach concepts that can trigger students intuition, in my experience as a student, will get more engaged students.

I have my old high school calculus text book and that is definitely how trig was introduced. The proof of Pythagorean Theorem using sin and cos was literally part of my undergrad CALC101 (I remember because my high school teacher demonstrated it for fun so it was a rehash)

When were you in Calc 101?

Before or after 2009?

https://forumgeom.fau.edu/FG2009volume9/FG200925.pdf

Before. It was clearly circular since the identity was assumed, the calc prof might have acknowledged this at the time.

My math teacher was teaching us Trigonometry without ever showing the unit circle, and the class was completely befuddling to me and other students. This was pre-internet, and I stumbled across the unit circle in a book and the meaning of sine, cosine, etc. on it - and was like a huge revelation to me and others.

I've enjoyed trig since, and one of the few math topics I continue to use to this day.

I remember just learning to code at a young age, but not knowing trig or seeing how it applied because I only knew it as being about triangles.

I had learned to use some game graphics library (specifically DJGPP with Allegro, coding in C) and I was trying to make a 2d spaceship game, think like asteroids, where you can turn the ship and apply thrust. I couldn't figure out how to take the angle the ship was facing and get a direction to move. Basically I needed to understand the unit circle. I eventually found what I needed probably on some old internet forums. From that point on, trigonometry really started making sense to me.