A part of my answer is also included in:
What computer hardware matters to a graphic designer?
- I look mostly at screen-resolution (the higher the better, although not for reading)
- Decent amount of RAM
- Consider batterylife vs performance (CPU and GPU) and weight.
I work freelance and typically a few days in a row with daily commutes. So I work at a lot of different locations throughout the year.
I pack a wireless mouse and sometimes also a keyboard.
Also Not strictly an accessory but nevertheless important on the go: batteries. Long trips with a lot of work, 2 fully charged batteries.
This is not relevant if you already have a energy efficient laptop with good battery life. (which mine is not) or if you need to merely transport the laptop to the location and not work on the move.
When I know I have to work at one location for a few days in a row I start requesting at once an additional monitor. That way I can get my eyes of the "small" 16,4inch laptop screen. Then I can use the external monitor as main and the laptop as secondary screen which is a lot more comfortable for any kind of work.
I have not been able to work with a tablet on the go or anywhere other than at home on my comfortable desktop PC with good monitor sizes. But that is mostly due to the kind of work. For illustrations and anything that requires the tablet, it's very important I'm 100% comfortable and in the "zone". So tablets on the go are out for me.
What I do a lot is keep a (mental)list of things I can't do while traveling and plan my time so I can do them later on. That also helps me to focus on the most important things.
On the go, as in the train or plane or ship I don't even get the mouse out. Exceptions are if I have to show or discuss something with another person. Then the wireless mouse is very handy. Mostly accompanied with verbal commands like "zoom in on that" and "let me move around a bit"
I have come to believe there is not a lot you can do while literally moving. I can only speak for myself, but my thoughts tend to wander A LOT when travelling so that more often than not, I end up snapping out of some very long train of thought (good luck remembering what those thoughts were) when the low battery warning pops up. Which is when I try to do everything in a couple of minutes just to avoid having to dig out that annoying charger and cords and go socket-hunting.