Although similar to Japanese maki sushi, our "leftovers" rolls don't require the use of special vinegared rice, just whatever we have on hand, be it white or brown rice, and occasionally even quinoa. The rolls are more akin to Korean gimbap, and in fact we prefer to use Korean-style seaweed (gim), which comes roasted and seasoned with sesame oil and salt. If you only have access to Japanese nori seaweed, you might want to mix a little sesame oil into the rice for additional flavor. As for the fillings, all kinds of vegetables are good, from steamed broccoli to blanched carrots, raw cucumbers, and pickled radishes.
A bamboo rolling mat makes it easy to create a firm roll, but in the absence of one, you can roll it up by hand. Sometimes we skip the rolling and slicing altogether and just scoop the rice and vegetables into a piece of seaweed, taco-style. You can dip the rolls in a seasoned sauce like yangnyeomjang if you want, but the seasoned seaweed is often enough.
There are no hard rules, only that you enjoy this simple, savory treat.
Buy online: Korean Seaweed at Amazon.com
Related: Five Ideas for Leftover Rice