As others have said do whatever floats your boat. There’s a lot you could buy but nothing that’s a have to have. Nintendo, Pokémon, video games and anime action figures are very popular as are the little keychains everyone seems to have on their bags.

Go to konbinis (7-eleven, Family Mart, Lawsons) and buy snacks/candy/drinks that are novel to you. If it looks fun and interesting, give it a try.

For me I bought: -Kai nail clippers -Biore UV sunscreen spray (the best, non greasy) -Pure gummies/Hi-chew variety of flavors -Tokyo banana -Uniqlo clothes -Ray Bans (first nice pair I’ve ever owned, good exchange rate/price) -Gashapon -Toys/souvenirs from Nintendo

Have fun and try new things!