If you use a digital camera, take several memory cards rather than one big one (then, if your camera is lost or stolen, you won’t lose all your pictures).
If you use rechargeable batteries, you will also need a charger (and a transformer if the charger cannot handle 220 volts).
Photography (or the use of flashes) is prohibited in some sites, for copyright reasons or to protect art.
Use discretion in what you photograph. Taking pictures of military personnel or facilities is often forbidden. Don’t take close-up photographs of Muslim women with covered faces or Orthodox Jewish men or women without first asking.