Regarding my trip to Thailand I particularly enjoyed the temples and the time I spent with the Akha tribe.
The Grand Palace and the Buddhist temples in Bangkok: Wat Traimit, Wat Pho and Wat Phra Keo are a must. You can see all of them in half a day if you are on a mission. They are all impressive and colorful. You will see the 3 meter high, 5.5 ton heavy Golden Buddha in the Wat Traimit. At 46 meters in length and 15 meters in height, the Reclining Buddha is the largest in Bangkok and illustrates the passing of the Buddha into nirvana. You can see it in Wat Pho. The Emerald Buddha it is in Wat Phra Keo. Palaces used for different occasions, like the Reception Palace, the Funeral Palace, the Throne Hall, the Coronation Hall, the Royal Guest House are all part of the Grand Palace, and used to be the residence of the Kings of Bangkok.
Chiang Mai and Chiang Rai had beautiful temples as well. I met nice people and had good inexpensive massages there but my favorite view and experience was staying with the Akha tribe. The accommodation on the hill was basic but the location was right out of a dream. Near water falls and in the center of green rich vegetation, the days and nights spent as an Akha tribe guest were relaxed and the time there was such a welcomed retreat for the mind and soul. Highly recommended.