One option would be the Fallkniven DC-3 or 4 diamond sharpener. #4 is slightly bigger, which makes it a bit easier to use IMO but it's still very compact. One side is 600 grit for sharpening and reprofiling and the other side is much finer for polishing. The Fallkniven DC-4 is the only pocket sharpener I've seen so far that can consistently put a truly great edge on any cutting tool. Knives, axes, scissors and everything else with a blade.

There are also a number of compact diamond sharpeners out there, both rods and cards (DMT is probably the best brand out there). They work great but lack the fine polishing capability of the Fallkniven.

Other than that, there are plenty of other choices. Artificial stone, ceramic... All work but some are fragile, some require oil and some don't work too well on super hard modern stainless alloys. Even sandpaper can be used in a pinch but it wears out very quickly.

IME diamond sharpeners are the best choice in the field because they will put an edge on anything, work fast, don't require lubrication, last long... It's just a simple, foolproof method.