Based on what I brought back to Taiwan in my suitcase after my first holiday back home (UK):
Big bottles of sunscreen (400ml) still do not seem to be as available here, although I have ssen them recently in a surfing B&B on the east coast.
Insect repellent with DEET. Many local brands promote themselves as not containing DEET, but then you get to Taitung railway station and a big board above the entrance advises travellers to use repellent with DEET. You can find it in some pharmacies for about NT$200 for 60ml. Icaridin/Picaridin is more expensive at about NT$500 for 100ml. When it comes to the awful tiny black midges, whose bites if scratched may last for days, clothes make a great defence if you are out of spray, because unlike mosquitoes they only only bite if they land on your skin.
Something I have not brought over but really wish I had: A beach windbreak. Some beaches, particularly those near Hsinchu, can get really windy here. Sometimes you get lucky and there is enough driftwood or old refridgerators around to make a barrier, but othertimes not.
The culture difference/ way people will react (this may be the same as Shanghai; I’ve only passed through. If so, please skip): Sometimes like they don’t see you, like Richard Ashcroft in that music video, which I put down to short sightedness. Sometimes people will avoid eye contact like their life depends on it. Sometimes people will stare, but then if they are not children, will pretend to not have been when you notice them. Some people will switch to English when talking near you “to put you at ease”, and some will see you and start talking about foreign things to each other. Sometimes you will get asked personal questions by strangers, for example, “Are you a virgin?” And sometimes random strangers will do things like buy you a cold drink if they are standing with you at a bus stop with no shelter on a hot day.
Some old people may seem to be in a parallel universe intersecting with this one in which it is okay to do things like cross a busy road wherever they like, push you out of their way, or walk into a shop straight up to the counter or into a train ignoring the other people waiting in line. And local people may choose to back them up in this behaviour. I put this down to them just wanting to do things the way they always did them before things changed. Related to this, people may attempt to interupt your transaction at a cashier as though their life depended on it and waiting for you would kill them. Having lived in Guangxi and Sichuan, I guess maybe it will seem a breeze compared to having to elbow your way to the ticket desk to buy a train ticket.
In social settings people seem more likely to state the obvious or make awful jokes instead of standing around awkwardly not knowing what to say. People will laugh or smile, seemingly at you, but most likely because they are nervous.
One thing related to all of these observations that I wish I had heard of before discovering it the wrong way: A local game/way of thinking called “He who takes it seriously, loses”. This may be related in some way to face, but (as you will know) there is much more to face than that.
With food, I wish I had known about the now obvious (island - duh!) fondness for fish sauce or dried shrimps for flavouring. If you love all this stuff then no problem. I’ve personally seen and ordered dishes called simply fried lamb or beef stew rice, plates of vegetables or rice triangles called zongzi, only to be unable to eat it due to the fish satay sauce or tiny dried prawns that were not mentioned anywhere, and have learnt to ask for their presence before ordering anything.
With traffic, one that I read but then forgot one time and still regret it: Never race away when the lights change to green without checking for vehicles running the red light across you, especially on a narrow intersection. Running red lights after they have just changed to red is habitual, and some people will speed through even if they admit they have seen other people have accidents doing the same thing at the same intersection.