Harajuku / Omotesando
Harajuku is not really the place for me, but I guess is maybe a bit of a rite of passage for people going to Tokyo? Buuut regardless, it is next to Omotesando, which I do really like! So I think it's worth bundling it all in together.
What to do
Kiddy Land
Great shop for taking the kids! There are I think 4 levels, that are broken into various themes so it's pretty easy to find you way to the things you like. Definitely do not go in with a pram if you can help it. That's pretty true of most shops in Japan due to the narrow aisles, but especially here as it is popular and the lift leads to a lot of waiting. Especially with people who are more unwilling to use stairs than unable 🙄.
Harry Potter Shop
Look, if you are a Harry Potter fan, you will probably want to stop by. Personally, I always find this kinda thing a bit underwhelming because obviously there is no magic, there is just merch and a loooot of people. But, there are some nice little bits in here and Easter eggs to find within the shop, so it's worth popping in if you are in the area anyway. I wouldn't come just for it.
Takeshita Street
This is the massive throng of people that you are going to feel obliged to throw yourself into. I'm not going to say don't do it, I'm just going to say don't be surprised if you don't like it. There are plenty of bright and intriguing things to see, I suppose, but I just cannot make my peace with it. If you want to get yourself giant rainbox fairy floss, then you are looking for Totti Candy Factory. Might I suggest that you have also stocked up on some wet wipes for this, from the Daiso also on the same street.

Yoyogi Park
I think this one might be a real time of year dependent experience. We went in the Summer, and it was quite hot, with large areas without shade but with manky grass. Grass is a bit of a rarity in Japan. There were definitely nice parts to it, so in more kind conditions I think it would be a good option.
Cat Street
No, there aren't heaps of cats here, it's just what it's called! It is an easy to walk street, that has lots of cool shops along the way though. The kids and I did enjoy a browse in B-Side Label, which is a sticker shop. Pip enjoyed a vintage clothing store, basically you will find things that interest you, and you can stoll all the way to Shibuya.
Food
I'm donut?
There are a number of these around the city, but there is one here so I will mention it. The fancy donuts are pretty good, but honestly, their basic one that is made with like pumpkin in it, a-mazing!
Hocuspocus Donuts
In Omotesando, there is this donut option. It's off Cat St, which I'm suggesting you walk down anyway, so you may as well seek out a donut and decent coffee! Many stairs though, FYI.
Rizlabo Kitchen
Fluffy pancakes that are not worth it! Out of the way, and while the toppings were nice the pancakes themselves were only ok. There are better options, like A Happy Pancake, that is not far from here.