Thailand additions forum

Hey all...
Me and my girlfriend are travelling to Thailand 26th - 16th Sept. I was hoping for a little advice on best way to plan a Itinerary which would include visiting temples and relaxing on some of the beautiful beaches. I really want to visit Cambodia, Southern Vietnam within the 3 weeks, am I being unrealistic trying to get all of this into 3 weeks? Also my girlfriends stepmum is from Thailand and she says the weather is fine this time of year is she slightly over exaggerating to make us feel better about it being the rainy season? Oh and my gf really wants to visit an elephant park is Chiang Mai the best place for this

Any advice would be greatly appreciated