1. Probably a decent sleeve or bag. Personally, will avoid any screen protector because the screen has a coating on it. It won't give you any problems until you damage the screen protector and when removing it, may damage the screen and it's coating. I avoid any protective casing as well, afraid it might mess up the cooling system, but that's just me.

2. Another set of charger, can be a third-party GAN ones. Reason being, I hate to repack chargers and cables so I usually have one set at home, another in my work bag or work desk. I have the Hyperjuice GaN 100W USB-C Charger and I've order another MagSafe 3 Cable.

3. I would suggest a USB-C Hub but the new ones have great I/O already. Maybe a USB-A to USB-C dongle for backward compatibility. I have this :
Anker PowerExpand+ 7-1 USB C Hub or something smaller like this UGreen Type C to USB 3.0 Adapter

4. A mouse? Depending on your usage. I have my old trusty Logi MX Master 2S for years, which is much better for my type of work. (Make maps, edit photos, etc.)

5. A hard drive for Time Machine, etc. I got the 512GB base model, and I use a Lacie 2TB Rugged Thunderbolt for my Time Machine. Got a Sandisk Ultra Dual Drive Go 128GB for quick file transfer between different machines. All of my work files are on DropBox since we share a lot between our team.

6. Maybe a Moft Laptop Stand - Was thinking about it for working on-site but most of the time I find myself using the laptop on my lap.

Apologies for the long winded post.