What's your list of things to bring when travelling?

I suppose this depends on the age of the your baby, how long the plane journey is and where you are travelling to.
We will be taking our 10 week old to Spain. So we will take some nappies but not loads as we can pop to the supermarket. I have also just bought some baby ear defenders as it will be fiesta season so it may be loud and I also read that it can help with noise on the plane.
I always take my own medicine for my other child who is older like Calpol as I don't want the added stress of trying to find something if they are under the weather.
I have traveled with my first quite a few times and found that the airport part was a breeze, you get to go through the family line at security, on the plane first so you get space in the overhead lockers. Etc.

Babies 4 months lost has been endless got uv sterilising bag, uv tent to chill in the day, long swimsuits and hats as under 6 month's can't do sunscreen, sterilising wipes, travel buggy and bag, extra bottles and teats, formula pots, nuby rapid cool, swim nappies, swim float with canopy, calpol, baby carrier for airport, formula as dont sell the same where we are going, ready made bottles and teats for airport, sun hats, vests when its hot and thicker outfits for evening, sleep sacks to try and keep routine unless too hot, endless dummies, blanket

You honestly really don’t need much. Don’t overthink it. You can buy almost anything that you need wherever you are going, nappies, wipes, most big band formula (although in some countries they go by different names so just google it) I’d recommend taking a few bottles, collapsible bucket, and some Milton sterilising tablets. We just went travelling around Sri Lanka for a month with our two babies and we literally took a small suitcase and we could buy everything we needed out there.

Baby carrier at the airport, push chair, car seat if you’re travelling in a car, that’s the big ticket items. presumably you’ll have a baby cot where you’re staying provided, so maybe a couple of extra sheets for that. We just tended to nap baby on the go or on the beach on a towel or wherever we were.
A thermos flask to take warm water out with you for the day to make bottles. That’s about it. A few outfits, you can always do a wash when you’re out there.