Walking shoes — the most important call
Osaka rewards walking. Dotonbori, Shinsekai, Tenma, Kuromon — most of what's worth seeing gets discovered on foot, usually across more ground than you planned. Comfortable shoes worn in before the trip make a bigger practical difference than almost anything else you pack.
Power adapters
Japan runs on 100V at 50/60Hz, with Type A sockets (two flat pins — the same as North America). Most modern devices handle 100-240V and just need a physical adapter; check your device's power brick before buying a voltage converter you don't need. If you're arriving from Europe, Australia or elsewhere with different plug shapes, a Type A adapter is cheap and essential.
Cash and a card that works overseas
IC cards handle day-to-day transit, but cash still matters at many restaurants, markets and shrines. A card that doesn't charge foreign transaction fees — and ideally works at 7-Eleven ATMs — alongside some yen in your wallet from the airport covers almost every situation.
The thing people forget: a small bag for rubbish
Public bins are genuinely scarce in Japan. A small zip-lock bag or foldable pouch to carry wrappers and convenience-store packaging until you reach the next convenience store or hotel bin is a habit worth building from day one.
Seasonal considerations
Summer means sunscreen, a compact umbrella and breathable layers — the humidity in July and August is real. Winter is mild but can turn chilly; a mid-weight jacket and a compact puffer cover most evenings. Spring and autumn are packing at their most straightforward.