How you wound up with the leftovers: It turns out that your beef stir-fry didn't have quite as much sauce in need of soaking up as you'd thought.
Use it now: Fried rice is the obvious choice; sauté chopped vegetables and/or meat, add the rice, cook until heated through and bind everything with a sprinkling of soy sauce. You can also stir the cooked grains into chicken, miso or tomato soup—or steal a trick from Julia Child and use them to thicken smooth soups without adding cream (an immersion blender will puree the rice and help distribute it evenly).
Use it later: Rice holds up well in the freezer for a month or two. Thaw it in the refrigerator or, if it's in a bag, give it a few good thwacks on the counter to break it up; then microwave it for a few minutes with a bit of water.