If you want to try something different you have to go to:
The House. It offers unique Fusion food (thai-western) in a nice setting on Moonmuang Road at the corner with the Soi 7.
To try Northern Thai food, there is the Thanam restaurant, on the river at the far end of Changklang Road. Nice wooden place, with live traditional music. Good for a romantic evening.
Behind the Anusarn market (the place if you want to eat seafood) there is the small 'Khao Soi Only' restaurant, serving just that, kha soi, a delicious mix of yellow noodles, crispy noodles, chicken, curry and other ingredients.