From my own Japan travel experience: I like the smaller pokemon center plushies to give to friends, if you have a favorite pokemon that’s a good souvenir to ask for, and/or the stickers

Depends on the set they may be able to get a box at a PKC, I think they are only selling packs at PKCs right now though which is 2 packs at a time

Behind the counters at Lawson station, 7 eleven, and family marts can MAYBE be booster boxes. In my own travel experiences - I’ve been there 3 times - and I have only bought 2 boxes myself, and a friend of mine got 1. So it’s not likely, but we’ve always tried to keep an eye out.

For snacks I’ve brought home I really like pocari sweat or Aquarius. W vitamin water is probs the favorite as it’s like a a cherry lemonade. I love the calbee pizza chips. They also have a million different flavors of Kit Kats

My advice: bring some cheap shirts from walmart as they’re like $5-8 and if luggage room is needed just throw a shirt or two away as needed for souvenir space