Indonesia is home to 167 active volcanoes, far more than any other country. Don't let this fact scare you, though, as most are dormant and what you see is most often their topography rather than spewing smoke. Some of the more accessible mountains for visitors are in the Bromo-Tengger-Semeru National Park and the Ijen Crater in East Java, Mount Rinjani in Lombok and perhaps easiest of all, Mount Batur, and Mount Agung, it's neighbour in Bali.
Hardly surprisingly in the world's largest archipelago, beaches are significant attractions. Aside from the obvious like Bali and Lombok, there are wonderful beaches in off-the-beaten-track locations, especially in Maluku, Nusa Tenggara and Sulawesi. In a nation of 18,000+ islands, the options are almost endless.