Being prepared
Since you can’t pack such as speedlights or strobes are out of the list straightaway. The first thing that you need is a diffuser. A diffuser will reduce the intensity of the light and create a softer light source ideal for head-shot and portrait type photography. The reason why you should prefer a softer light source against more harsher light is because harsh light will create intense shadows around the eyes, below the nose (depending on the time of the day you shooting and the angle of the Sun from the horizon), around the neckline and also enhance any imperfections of the skin. Softer light is conducive to professional head-shots, skin imperfections are cancelled out and so are deep shadows.
The next thing that you need is a reasonably fast lens. The 70-200mm is a great portrait lens. Some photographers prefer the 135mm while others prefer the 105mm. Pick whatever gives you the best results. Using a 70-200mm lens allows you to cover the essential length range for portraits and head-shots. You really don’t need a fancy lens such as an f/1.8 or even an f/1.4. Even at f/2.8 you will get great Bokeh for your head-shots. Having said that, the Nikkor 135mm f/2 DC is a personal favourite. It’s built like a tank and creates beautiful images. A comparable lens is the 105mm f/2 DC, also from Nikon.