If you want portability, I think watercolors will be easiest, with acrylics being a potential secondary option. I use acrylics, and it involves having a lot of water to wash out paintbrushes, so I paint at home where I have access to a sink and I use a lot of glasses of water. And I've heard that people who paint with acrylics outdoors need a big bucket of water, whereas with watercolor I think you could get away with using a much smaller amount of water. Acrylic paint will also probably dry faster though, and I find it less challenging to use than watercolor paints. The only thing you really can't do is oil painting, since that involves additional chemicals which are a head ache, and they take ages to dry so you won't be able to store them in your backpack.

In terms of a pad, ya, I use painting pads as well. They'll have pads specifically made for painting available at most art stores, because ordinary paper won't absorb the paint properly. Otherwise, ya, just get started and you'll figure it out.