It is definitely rainy in Indonesia in October; however, the storms typically only last for a few hours, usually in the late morning or early afternoon. They sweep in suddenly and forcefully and then clear up fast, returning to sunshine and blue skies. As long as you're someone who isn't too bothered by brief showers—and you have a good rain jacket to bring along—it shouldn't prevent you from annoying your vacation. What's more, Bali—one of the most popular islands—is less affected by the storms (although it certainly gets its fair share), so if you stick to that region you won't feel the brunt of it as much.
Temperature-wise, it's warm and pleasant. Bali and Sumatra hover between 84°F and 90°F during the daytime and drop to roughly 73°F to 77°F at night. Java is hotter, averaging daytime highs of about 95 to 99°F (and only cooling to about 78°F in the evening).
Crowds and Costs
October is a great time to come to Indonesia if you're looking for smaller crowd sizes and cheaper prices. Although it's by no means empty—or even as uncrowded as later in the winter—it's nevertheless much better than in the middle of summer. It's a transitional time when the tourists begin leaving and locals shift into low-season mode. It's a good time to snag deals on hotels and airfare, and you won't find as many people at all of the tourist attractions.
Where to Go
Bali is less affected by the rains than some of the neighboring islands and for that reason, it's one of the best October destinations. Not only that, it has much better infrastructure due to the large amount of tourism it receives so you're less likely to encounter washed out roads and power outages (which can sometimes be issues on other islands).
Java is one of the next best options. It's home to the capital city of Jakarta which gives it higher-quality infrastructure, too. If you opt to head that way, stick to the eastern part of the island which gets considerably less rain. Destinations around Surabaya and Lumpur will make good choices.
Another option that can be ideal for travelers with a sense of adventure is to head to the Moluccas Islands (also called the Spice Islands). The rainy season is different there and it won't be nearly as wet. Just keep in mind that these islands are more remote and won't offer the same degree of amenities as you'll find in the more heavily touristed destinations.
It's best to avoid places like Nusa Tenggara, Lombok, and Flores which will be inundated with water.
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What to Do
In Bali, the southern beaches get more of the rain so stick to the central and northern regions where you can. Check out the town of Ubud, a quaint town that's famous for meditation and yoga retreats. Take a class at the Yoga Barn or wander through the beautiful Monkey Forest (Mandala Suci Wenara Wana) where you'll be greeted by eager Balinese long-tailed monkeys. The rice paddies outside of town will be green this time of year so it's the perfect season for a bike ride through the countryside or a long walk through the fields. Or, just up the road at the Celuk village, visit the traditional silversmiths where you can shop for souvenirs or watch them work.
In Bedugul, just north of Ubud, check out the Ulun Danu Temple that sits directly on the water at Baratan Lake. It's a stunning piece of architecture that pays tribute to the goddess of the lake. Take an early morning tour of the temple to watch sunrise which is actually best during the rainy season. Adrenaline-seeking tourists can visit Bali Treetop Adventure Park or more mellow travelers can enjoy a peaceful stroll through the Bali Botanic Garden.
In Java, the volcanos will be wet and visibility may be low so it's not a great time to climb Mt. Bromo or Mt. Ijen. However, the island offers lots of other interesting activities. Head to Surabaya, for example, where Indonesia's second-largest city offers a host of urban activities such as museums, shopping, food, and nightlife. Another option is to visit the Borobudur Temple complex outside Yogyakarta or the Candi Prambanan complex near Borobudur.
Events in October
Maulidur-Rasl (Muhammad's Birthday): This scared Islamic holiday, which is celebrated by Muslims worldwide, marks the birth of the Prophet Muhammad. In Indonesia, it's a public holiday called Maulidur-Rasl. For Sunni Muslims, it occurs on the 12th day of the Islamic month of Rabi' al-awwal and for Shi'a Muslims, it takes place on the 17th day.
Traveling to Indonesia in October? Check out this great itinerary
Highlights of Bali - 8 Days: If you'd rather have someone else do your trip planning, this 8-day excursion is an excellent option. It's perfect for October vacations because it forgoes the wetter islands that aren't any fun this time of year in favor of Bali which typically gets less rain. Highlights include a trip to the Tanah Lot water temple, a cooking lesson in Ubud, and a a bike tour through the countryside.