Depending on what kit I have with me, I use a small thumbed sized whetstone that is now about 20 years old. I also have a newer whetstone which is a little bigger, however I don't really care for for it...and for no real reason expect that I probably prefer the old stone better.

In the event I forget to take the whetstone, almost any smooth surfaced rock (river rock is best) will work perfectly fine to sufficiently touch up any blade.