1. Small size. It's not easy working on a seamless adult size sweater on the move once you get a decent amount of it worked. Socks, mitts or charity squares are much more portable.
2. Circular needles (or a crochet hook, at a push). I work in-the-round projects almost exclusively on one circular needle using the Magic Loop technique. It means if I drop anything, it all stays together. I can't imagine how frustrating it would be to have one of a set of DPNs drop down a drain or down the gap between the train and the platform.
I'm not much of a crocheter and tend to restrict myself to charity squares, but at least it's just one pointy stick to drop!
3. Centre-pull yarn ball. This might not be the most obvious, but a centre-pull cake sits still when you pull more yarn out. If you're pulling from the outside, the thing is inclined to roll about. You might get away with it if it's a cake from a ball-winder as the bottom is flat so it might sit still, but an actual round ball is going to be off across the floor as soon as you look at it!
4. Small drawstring project bag. Seeing me using one of these was the point my mother-in-law released I was a Knitter (with a capital K). With a drawstring bag looped over my wrist, a centre pull ball inside it, and a sock magic-looped on a circular needle, I can walk and knit socks at the same time!
5. Memorise the pattern. Depending on the circumstances, you might get away with a small printout, or a copy of just the chart if it's a small on, in the bottom of that drawstring project bag if you don't need to refer to it too often. Ideally though you want something you can work on with needing to check the pattern whether it be just stocking stitch or garter stitch, or a simple repeat on a stitch pattern. Plain vanilla socks (just knitting round and round and round, except for the heel and the toe), charity squares or dishcloths (just back and forth knitting, or crocheting round and round and round), a simple hat, all things that can easily be made while moving about.
So there we have it. There are other things I could list, like not needing too many notions - I don't want to be shedding stitch markers all the way down the high street, and I lose enough tape measures without taking them out and about with me - but these are my Big Five for the Knitter On The Go!!