It’s surprisingly easy to get from Chang Mai to Koh Lanta in the same day, even though they are at opposite ends of Thailand
Even though Chiang Mai is at the opposite end of Thailand to Koh Lanta, in the far north, it’s actually remarkably easy to get from Chiang Mai to Koh Lanta in a day if you don’t mind spending a bit of cash. Essentially you can catch an Air Asia flight from Chiang Mai to Bangkok in the morning, then Bangkok to Krabi in the afternoon (both one hour flights) and get a Fast Speedboat Transfer from Krabi Airport to Koh Lanta or a Minivan Transfer to Koh Lanta. The flights arrive in and out of the same terminal so it’s simply a case of getting your bags and checking in again for the flight onwards.
You can find the cheapest possible dates to fly Chiang Mai – Bangkok and Bangkok – Krabi using Skyscanner which compares the fares between various days to show you the cheapest time to fly.
Alternatively, you can take the train or bus from Chiang Mai to Bangkok, and then get the overnight bus to Krabi from Bangkok – or fly one leg of the journey and then train/bus the other leg.
You can also check hotel availability on Koh Lanta for your chosen dates.