I could include Chiang Mai Erotic Gardens, if it is still going. There's an interesting collection of Thai proverbs in Thai and English at the Doi Saket temple.
Chiang Rai is a jumping off point for so much more, Mae Sai markets, Phu Chi Fa, White Temple, Blue Temple, Black House, Doi Mae Salong, Doi Mae Mon, Chiang Saen Skywalk, the Mekong, Doi Chang and Wat Sang Kaew Phothiyan.
I suppose with time Chiang Rai will become as congested as Chiang Mai.