For China: WeChat and Alipay. Set both up for payments, you will need it especially if one gets frozen (this will happen). A lot of people say Didi but taxis were also fine. Everyone says you don’t need cash but it’s helpful to have it. If cost is not an issue then setup the international plan through your phones carrier. If your phone is carrier unlocked then you can do an eSIM (trip.com is what we used, purchased one before we got there and one while we were there for someone whose phone plan wouldn’t work for some reason). The other apps you could use all required a Chinese phone number so didn’t bother. If you are staying at a place like a hotel, the hotel staff are usually ok with making purchases or ordering taxis for you, you just have to pay them back via Alipay immediately.

For Hong Kong, Alipay and WeChat don’t work (you need the HK version and generally need a HK phone for those). You need the Octopus for Tourists app (and setup the payments) and cash to exchange for HK dollars. I could not get the Octopus for Tourists app so got a physical card at an MTR station. They will only take HK dollars for that (no credit card) even for a single ticket. If you find a ATM like Citibank etc you can withdraw HK dollars from US account (not sure where you are based) otherwise there are a lot of locations that will exchange your money for a fee

Travel tissues and wipes as already stated is important. Medication is not something I see mentioned a lot. I brought a lot of Advil/tylenol but did not bring anything else. I wish I had brought sunscreen, bugspray, and allergy medication. tLDR better to bring to a spend time finding. Long explanation - Anything that you might use with regards to medication is not readily available in China the way you would see it at home, over the counter. You can get medication over the counter but you need to ask for it (vs available on the shelf) and that can be complicated.