There's nothing really special about the town of Huai Khrai itself and no need to spend very long there. But the surrounding area is made for traveling round, and discovering new things, and is full of surprises like lake Sao Hin and the cave of Tham Pla, a little to the north.
A tip for those who like trips that are a bit out of the ordinary : south of the town I found Wat Tham Pa Asha Thong, the monastery of the golden horse, founded by an ex-boxer who came to the area to find peace and quiet. Listen to its story - it's incredible!
The main attraction of Haui Khrai is the Royal Palace of Doi Tung which was built by the mother of the King of Thailand as part of an attempt to counter opium production in the region. Today the Palace has been transformed into a museum which is well worth the trip, even if it's just to admire the gardens and the architecture of the buildings.