Bali
Three or four completely different trips sharing one island.
The best street food on the planet, and a transport system that makes the chaos navigable.
Bangkok is loud, hot and much easier to enjoy than its reputation suggests. The BTS and MRT get you above and below the traffic, the river boats are a genuine pleasure, and the food — from Michelin-listed street carts to malls with better food courts than most cities' restaurants — is the reason to come.
Most people give it two days on the way to an island and regret it. Four days lets you do temples, markets, a river day and a night market without sprinting.
The cool dry season — still warm, but humidity and rain drop enough to make walking a temple complex bearable.
A workable shape rather than a schedule to obey — two fixed things a day, and room to change your mind.
BTS Skytrain and MRT for anything they reach, river ferries along the Chao Phraya, and Grab or Bolt for the rest. Metered taxis are fine if the meter goes on; tuk-tuks are a novelty, not transport.
Street food is the great group equaliser — everyone orders separately for a few dollars and nobody's arguing over a shared bill. Fix one meeting point per day; phones die fast in the heat.