The year-round temperate climate, the challenging but not daunting terrain, the fascinating culture, and the layers of history at every turn make both Bali and Lombok wonderful places to explore on foot. The islands are also small enough that you'll never feel like you're in the middle of nowhere and the prospects of getting totally lost are pretty limited. Hiking is best (and safest) during the dry season (Apr -- Oct).
Most peaks, if they are your goal, are scale-able in less then 8 hours, and many can be done in less than 4. The one great trek that takes you furthest from regular habitation and requires careful preparation is Mount Rinjani on Lombok. This challenging excursion will take 3 or 4 days, leaving from Senaru, and most definitely requires a guide.
The majority of other mountain treks necessitate a guide as you will want to be at the summit by sunrise. You therefore will need to head out in darkness at any time from 2am onward. You should be provided with both a head and hand torch. Sunrise, when you get there, is quite a magical and a sublime experience -- weather permitting. It is not a wonder that the Balinese consider the peak of Mount Agung the island's most sacred spot. Indeed so much so, you are not allowed to climb at certain times of the Balinese year. Climbing Mount Agung is a pilgrimage for all Balinese and at some stage in their life, they will undertake it at least once.
Trekking agents for Bali include Bali Adventure Tours (tel. 0361/721480; www.baliadventuretours.com); Clipper (tel. 08/123802675; www.clipperdiscovery.com); Sobek (tel. 0361/287059; www.balisobek.com); and Gede from Bali Sunrise Tours (www.balisunrisetours.com), an old original.
For Lombok, many tour agents and trekking offices are based in Senggigi, Senaru, Sembalun Lawang, and Sembalun Bumbung. Lombok Rinjani Trekking (tel. 0370/6686625; www.lombokrinjanitrekking.com) offers fully equipped treks. Ensure you check in with the official Rinjani Trek Management Board (tel. 0370/641124 or 08/11390047; www.lombokrinjanitrek.org) for all aspects of trekking and guides on Mount Rinjani.
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.