If you are in Bangkok, make sure to visit the china town district. I liked the gritty, dystopian feel and also the great food of course
Night train to Chiang Mai
The easiest, and also most comfortable (if you know what to look for), way to get from Bangkok to Chiang Mai for me is the night train. They run at least once a day and can be completely booked in high season, so make sure to book a few days in advance. There are special ticket offices in the train stations for foreigners that will help you to get the right ticket. The most comfortable beds are the lower ones in air-conditioned wagons. The upper ones get the full blast of the AC and don't have a windows. Wagons without AC can get, unsurprisingly, very hot, so make sure you have the lower bed as well to get at least some cooling from the window.
Make sure to take a trip to the Don Inthanon national park. The most remote parts are only accessible with a guide but you will get to see strawberry fields, coffee plants, beautiful waterfalls and tiny villages that seem mostly untouched by modern technology.
Most people living there seem to live from agriculture and are not ethnic Thai but are assembled from various groups, for example the Karen people that originate from Myanmar and still speak their native languages.
A small roastery prepares fresh coffee from the local hills. The beans were first ground fairly coarse and then put in a basket made of fine fabric. Hot water directly from the fire was then poured over multiple times to produce a mokka like coffee that I was totally unable to appreciate because of the heat.
In a tour de force we made it from Chiang Mai to Ko Lanta, a small Island not to far from the tourist hub Phuket. Take a plane if you can.