A long charging cable. Appointments can take a while, or rather, they can run late.
Lip balm
I used Mytrusty sunflower cream on my radiotherapy area - it's not a strong fragrance and was developed by the NHS in Salisbury
Warm socks
Button up cotton pyjamas - I had shorts and shirt - it was very hot in the hospital
A text/WhatsApp rather than a call, they don't always arrive at a good time.
Company when they are ready (and make the cuppas yourself!)
An offer of a lift. Sometimes follow up appointments can be more frequent than anticipated. I had a mastectomy and ended up going 2-3 times a week to get the fluid out (it was around 500ml in one week - admittedly, I was big of nork, but never expected that!)
Zip front bra with no underwire. I still wear the ones from Sainsbury's, about £10 and come in s/m/L/XL and are soooooooo comfy, while providing some decorum when you need to undress.
Or, just ask. They may have these things from others. Sometimes we just want someone with us, not necessarily even saying anything. It can mean the world.