One thing people forget when coming to Thailand on a holiday is the amount of internal travel time it takes to move to different destinations.
If you plan on visiting Bangkok, Chiang Mai and then off to the islands, you need to account for the travel times moving between each location.
Flying is the fastest method, however, getting to the airport 2 hours early for your flight, the flight time and the travel time on the back end all eat up time of your holiday.
If you take a bus or rail, the times can be ridiculously long moving between destinations. 12 to 24 hours is not out of the question depending on how far you have to travel.
Make sure you budget time, once inside of Thailand, to move around to the destinations on your itinerary.
This could eat up 2 days or more of your holiday.