The bag in the hold gets delayed. Sometimes it gets opened. I still check a suitcase on a long trip. I just never put the things that would stop the trip inside it.
1. Medicine
Anything I take every day stays in my personal item. A pharmacy in a new city is not a plan. Keep it in the original box if you can, so nobody has to guess what the pills are.
2. One full outfit
A top, underwear, socks, and something warm enough for the evening. If the suitcase misses the first night, I can still leave the hotel in the morning. I learned this the hard way, and I have not skipped it since.
3. Charger and a power bank
A dead phone is a lost map, a lost ticket, and a lost hotel address. The cable is small. It does not go in the hold.
4. Passport, cards, and a little cash
This one sounds obvious. I still see people drop the passport into the checked bag at the desk because their hands are full. I put mine in the same pocket before I leave the house, and I do not move it.
5. The camera, and the card with the photos
The camera can be replaced. The photos from the trip you just took cannot. Cards and the camera ride with me. Always.
6. Jewellery and anything I would actually miss
If I would be upset to lose it, it does not go in the suitcase. That includes the one nice necklace and the hard drive, if I am travelling with one.
7. The paper for tomorrow
The first hotel name, the next flight, the tour voucher. Screens die. I keep one paper copy where I can touch it.
Weight is a separate problem. Airlines are strict, and the fees add up. I wrote about the truth about excess baggage fees if you want the money side of it. And if the trip is a cold one, the small things in your jacket pocket matter more than another pair of shoes. That is the same idea as this list. What you need on day one stays with you.