A friend asked me this today and i thought it was an interesting question.
I miss 2 things -
Dad reading The Lord Of The Rings to both my brother and I (must have been about 7 or 8).
Teleportation - When i fall asleep on the couch now I wake up on the couch not in my bed like i used to.
Playing with my Matchbook cars (Hot Wheels)
Actually there is a lot more than just that, but it starts getting too serious and dear to me. So I’ll just stick to the Matchbox cars 
Summer vacation that seemed to last for years!!! Now I blink and my measly time off is over. We used to stay in a gas lit, no indoor plumbing cabin in upper Michigan that was in the middle of nowhere on the shore of a lake. I remember the taste of the cool spring water and how good food tasted eating every meal outside. I learned to swim in that lake as well as paddle a canoe and NOT tip it over. Eventually. 
Oh, and getting up without every joint popping and cracking. When did that start?
Riding bikes all day long, every day. And not feeling it in my ass for a full week.
NO BILLS lmao and sports i loved playing team sports and boxing , no responsibilities
Disposable income (aka free allowance)
The sheer ignorance of how hard life can really get sometimes. Almost everything was a new experience and exciting. Riding my bike around the neighborhood with my friends without a care in the world. I just hope I can give mine the same great memories I have.
The magic of christmas actully. Who was santa e.tc i did not expect to sew my mother fly sky high when she was filling my christmas sock and pulling the rope that was tied to my toe so i would wake up and meat santa. Buth hey still to this day my mother has not gotten that scared. So i guess it somewhat funny
I miss my fearless mindset and the idea that i could become a ninja, crawling in trees, jumping from roofs into sandboxes and so on 
And i miss all the wonders in the world that seemed almost magical.
I miss observing and playing with all the new and emerging electronic technologies. And yet sometimes i wish that never happened, sometimes…
We played board games, we played in the dirt and sometimes we slept outside under the stars. And when these new technologies started invading our homes we could just turn them off and walk away. We could rely on our neighbors because they needed us as much as we needed them.
I miss the simplicity of it all…
When you could turn on regular TV and see good boxing matches. Oh where has the sport gone?
Playing “kick the can” with entire neighborhood on the weekends as soon as it got dark.
The huge willow tree in my front yard i could climb so high and not be seen in
Not having a fear in the world of strangers , violence etc
As mentioned above, NO BILLS would be tops, but also, leaving the house in the early morning, with the only stipulation to arrive before dark. No cell phones, no internet, no pagers, no hassles. Ahhh the simple life.
I miss free time. It seems like there was more time in the day to do stuff. Now evenings fly by and don’t get me started on weekend time.
Sat morning cartoons
Who’s brilliant idea was it to take them off the air
Everybody had like 5 kids, so there were always enough for Teams. When my kids were growing up everybody was having like 2 kids …no neighborhood teams (football baseball basketball etc) …oh and yards big enough for those games
I miss my little play swamp and squishing toads against trees.
I miss always waking up to a hot breakfast mom would have ready…as opposed to being the one who cooks and subsequently washes the dishes (well, loads the dishwasher lol)
Mostly tho, like it seems everyone is saying, I miss the simplicity of it all. More specifically, the endless summers playing with the neiborhood kids and Saturday morning cartoons religiously with the above mentioned hot breakfast mom would have ready or a magical bowl of cereal. Your parents tell u, “don’t be in a hurry to grow up” and before u kno it, you’re grown up (or pretending to be) and wonder when it passed u by.
Too many things to mention but I do miss the simplicity of life before the advent of technology where we had to interact and have conversations face to face and also make our own fun without the aid of a screen.
I miss being young. Back then everything was exciting, interesting and life was endlessly entertaining.