Are you going by car or motorbike? CK Crew (a group of 6 friends in Chiang Khan riding motorbikes) did a trip around Nan last year. If you want to follow a scenic route I'd recommend going to Nong Khai first, then follow the river to Chiang Khan and Na Haeo, then backroads up to Nan. From there you could follow the route KhunLA recommended. Very nice indeed.