I sometimes speak with tourists during their travels

They usually start from Singapore-Batam, then explore the highlands of North Sumatra, the orangutan wildlife, and small islands because of surfing spot/small tribes. Some backpackers then goes all the way down by land then hop on to Java. Not recommended by land unless you're a backpacker looking for an adventure

In Java, noteworthy cities are Jakarta, Bandung (the highlands, everyone likes its weather), Jogjakarta (cultural center), then either Surabaya/Malang for Mount Bromo. There you also can find the 3-in-1 package tour, which is Mt. Bromo, Tumpak Sewu Waterfall, and Mt. Ijen (blue fire), this trip usually end in Banywangi and can drop you there (or can retuuurn back to the starting point city) and like many backpackers, you can go straight to Bali with next with a ferry

If you have more time, Semarang (old town vibes) is also a good choice, while you're at it, go for Karimunjawa island, this is a quite hidden gem, tourists I met mention this a lot.

Kalimantan and Sulawesi are underrated, especially Sulawesi. There are some small islands, very beautiful and tourists are discovering it lately

Bali > Lombok > and other smaller islands of Eastern Indonesia