Suggestions for ideal itinerary three week trip to Malaysia and Sumatra

We're travelling with two kids (10 and 13) who love adventure and new things, and we're eager to see the cities of Malaysia, and nature and daily life in Sumatra. We're obviously not going to trek up a volcano for three days, but otherwise would love to hear what destinations should be part of our plans, and how many days we should spend there.

We have time, we have finances, and we are used to holidays where we do a lot of sightseeing and travel. We've mostly camped in Europe, US and Africa, but have never been to Asia before.

We'll probably be flying in and out of Singapore, and travelling in July/August for at least three weeks.