Gunung Leuser is one of the best places in the world to see Rafflesia in bloom — usually Rafflesia atjehensis or Rafflesia rochussenii, both species endemic to northern Sumatra. The plant is rare, hard to predict, and at any given site only one or two flowers are likely to be open. Local guides keep close track of buds in known patches near Ketambe and Kedah and can usually tell you on the morning of your trek whether anything is currently in bloom.
If you are determined to see one, build a few extra days into your itinerary and ask in both Ketambe and Bukit Lawang. The flower is striking once you find it: massive, fleshy, mottled red and pink, sitting flat on the forest floor like a bizarre piece of fungus. The smell is genuinely unpleasant up close. Do not touch the petals — they bruise easily and a damaged flower is a wasted reproductive cycle for a plant that may only bloom once every several years.