The Blue Diamond on Soi 9 just of Moon Muang Road (Chiang Mai )serves a great selection Thai/western/vegetarian food in a lovely little garden. I usually go here for breakfast or lunch but it is also open for dinner. If you have a sweet tooth it has a big selection of cakes and desserts and they have a health food shop that sells everything from muesli, herbs, teas, seed bread to fresh avocado pears.
Prices are reasonable and although its main emphasis is on healthy food they also have plenty of unhealthy options. The Thai food isn’t really authentic, more tourist style, but the western and vegetarian options are spot on!