Chiang Mai and Chiang Rai would be the places to get close to nature, especially the latter. I wouldn't worry too much about the weather in May-June. I've gone to Thailand at that time of year every year since 2005; while brief but heavy rain showers are common, they don't last all day and shouldn't interfere in activities or sightseeing too much. You might also consider places along the east coast, such as Koh Samet or Koh Chang.

You could also plan on cycling trips heading out from Bangkok.

For me, 19 straight days in Thailand would be a lot, and I would most likely pick someplace else to go for some of that time. But Mr. Megatop lives in Bangkok and I've been there quite frequently in the last few years. So whether to recommend going to Bali might turn on how much time you and your partner have spent in Thailand recently.