The short answer is: it depends. The long answer...is longer. And starts with 'it depends'.
It depends on who is going, and for what. It depends how long it is going to take, and the length of your journey.
It starts with the bag:
Something light, yet strong. Something that will allow you to have at least one hand free at all times. Something you will be able to carry or drag or pull when you are very, very tired. Something that won't fall off your shoulder at the worst possible time. Something that is the right size to hold your stuff, plus the chemo patient's stuff.
Backpacks and across the body bags are good. I'm sure a wheeled bag of some kind would also work, but you have to consider terrain. How are you getting to the hospital? Cars? Buses? Trains? Subways? What about stairs?
What stuff do you take to Chemo?
Below is a list, based on stuff I've packed while being a cancer sherpa, and what others have packed when they were sherpa-ing:
1. Directions and maps.
To the hospital. Or, in some cases, to the part of the hospital your patient is going. Sometimes Hospitals are HUGE complexes of buildings. Know where you are supposed to be going.
2. Change.
Change for parking (although folding money will probably work, too), change for vending machines. Change to toss into a wishing well, maybe. Carry coins.
3. Kleenex.
Hospitals tend to have those tiny KimWipes thingies. I have disliked those since the early nineties. Sadly thin, half-width approximations of facial tissue. Bah!
5. Something to write with.
Two pens, one mechanical pencil and a black fine point sharpie. You never know when you will have to fill in a form, do math or label something. They don't take up much space.
6. A Swiss Army knife.
You can use one to trim the dangling end of a hospital id bracelet, or open a packaged 'sandwich' from the cafeteria, or put the tiny screw back into your glasses. Just don't try to get on an airplane with one.
7. Reading material.
Yes, hospitals have magazines. Hospitals also have drug resistant bacteria outbreaks. No one sanitizes the magazines. Bring your own. Your patient is on Chemo, and as such, has a weakened immune system.
8. Something else to occupy your time.
A deck of cards or a portable DVD player (or smart phone) with a headset. Or two. Cross words. Sudoku. You and your patient are going to be in the Chemo Ward for a while. I don't recommend bringing knitting, crochet or cross-stitch. Again, germs. Yes, the chemo ward will be nice and clean, for the most part. It's the people who wander through the hospital who are the problem.
9. NOTHING OF VALUE.
OK, yes, I just said DVD player up there. Here's the thing; at some point, you will need to go to the bathroom. Part of your job as Sherpa is to watch the patient's stuff. If it's not something that you are comfortable leaving unattended, don't bring it.
10. A beverage or two.
Water, juice, ginger ale, what ever. Your patient may have special stuff that you will need to carry, or perhaps the hospital will provide water or juice for them. You will need to carry your own provisions. Be aware that you might not be allowed to consume food or drink in the chemo ward. If this is the case, be nice and take your snack outside without fuss.
11. Breath mints or gum.
Some people are very sensitive to scents when on chemo. If your patient is one of these people, you the Sherpa should be aware of how you smell. Ask. Encourage a blunt reply.
12. Travel Eye mask and ear plugs.
This one really depends on your patient. Perhaps they will want to nap. Perhaps not.
13. Lip balm and hand cream.
Hospitals are dry. You will wash your hands a lot.
14. Hand sanitizer.
Yes, Hospitals have the stuff everywhere. But what if you stop at a coffee shop on the way?
15. Large plastic zip-top bags, like Ziploc.
I always pack at least one. You never know.
This list is not complete. Or maybe it is. Or maybe there is too much stuff on it.
It is a starting point. Pack what you think you will use. If you don't need it, don't take it next time. Revise as you go.
I do recommend keeping the support pack, or chemo-bag ready with the stuff you'll need for the next trip to the ward. Top off whatever's depleted when you get back. Get in the routine of having the bag always ready and you won't have to rush around at the last minute.