For soft lighting while on-the-go you can easily take the Flash Disc Portable Speedlight Softbox from Fstoppers. The softbox measures about 12" in diameter and uses silver lining in order to more efficiently direct light. It works with most shoe-mounted flash units and uses an elastic band on the base to hold it secure. Also, it has a patch on the face that will prevent hot spots from the speedlight.
For additional functionality while shooting, the reverse side of the Flash Dish features a gray card with white, 18% gray and black panels for simpler color correction in post. And, for convenience, it folds down to about 4" and packs into an included carrying case.