When visiting any of the volcanic areas, understand and be respectful of their active nature. Never take unneccesary risks.
Temperatures in the high parts of the Bromo-Tengger-Semeru National Park can get close to 0 °C at night - come prepared.
talk
People in East Java speak Indonesian as well as Javanese which they mix together at times. A significant minority also speak Madurese. English will be understood and spoken at large city hotels and at obvious tourist destinations.
For all place names, beware that in the local East Javanese accent, "A" and "O" are largely interchangable: often the official spelling uses "A", but the locals pronounce it "O". Hence a resident of Surabaya is locally an arek Suroboyo and Cemoro Lawang and Cemara Lawang are the same place.