While I'm generally not much for guidebooks, Indonesia is one of those places where a good guide will definitely come in handy. Most hotels do not have a wealth of free information for travelers, as you will find in other parts of Southeast Asia. Since it is such a big country, a guide can be a useful tool in planning your trip. If you just plan to visit Bali, you may be better served by buying one of the specific Bali Guides available.
Indonesia is one of the premier destinations for outdoor enthusiasts of all types. Here are three guides for those looking for things to do on the water and under it as well as on dry land.
Diving Southeast Asia: A Guide to the Best Dive Sites in Indonesia, Malaysia, the Philippines and Thailand (Periplus Action Guides)