There are actually two high level routes from Villafranca. One, to the north of the "official" Camino, goes through Pradela. It's well waymarked and popular, with some nice views.
The other, via Dragonte and San Fiz de Seo (among other small villages) runs to the south. I tried walking it twice, in 2002 and 2004. Each time, I failed to complete the route. The waymarking was nearly non-existent, despite posts I'd found online prior to my trip indicating that the route was freshly marked. Perhaps someone on here has tried it in the last few years and can report on current conditions.
Without new markings, try at the very least to get a map in town before attempting this route. People are very hard to find in these small villages, so you can't count on help up top, and there are many different trails and dirt roads winding through the hills. I don't regret the two failed attempts - it is a very nice walk - but it probably added at least 10km to my trip and meant that I didn't make it to O Cebreiro that day. I've never met another pilgrim who tried the route.
In 2006, I walked to San Millan de la Cogolla from Najera. It's a little tricky - I used the advice in Gitlitz/Davidson's book, following the river out of Najera and then walking towards the depression in the distant hills, winding through farm fields. To reconnect with the Camino afterward requires a lot of road walking, en route to Santo Domingo de la Calzada. Alternately, you could just take a taxi back to Najera; the tourist office in one of the monasteries can assist.
The monasteries of San Millan are really nice. I particularly like the smaller one up on the hill. It's a peaceful setting, lots of trees and grass, and largely unvisited. San Millan is considered by some to be the cradle of the Spanish language, with perhaps the oldest written document. They reward pilgrims who make the trek with a copy from that Codex. It's a cool little reward.
Shortly after Sahagun, the Camino splits. The majority of pilgrims take the southern fork, following the "Camino real," which passes through a number of villages along the way, with at least a couple of albergues. I did it once and got turned off by the artificially constructed pilgrim's way, which I remember as "tree-tree-bench." Since then, I've followed the Roman road. It's longer and only passes through one village, Calzadilla de los Hermanillos, but I find the scenery to be really nice (on the first half in particular). Plus, there are a few well preserved patches of the old Roman road.
Another place the Camino splits is after Triacastela. The shorter route goes to Sarria. Don't be tempted! Though that route is nice enough, you don't want to miss Samos. The monastery is impressive in its own right, but the whole surrounding environment is stunning. Well worth the extra kms.
There are lots of things to visit along the Camino too, if you're willing to go an extra km or two. In Estella, you can find some limited castle ruins on the hill above the south-side of town. Even nicer are the castles in Castrojeriz and Vega de Valcarce. I've never seen more than a couple of pilgrims in either of these, but they're well worth a visit - for the scenery and the solitude. You can visit the Pazo de Ulloa, 1km south of the Casa Domingo in Casanova, though it's only really of interest if you've read Bazan's book (which I recommend, as a great, albeit fictional, example of rural Galician life).
One I never made it to is Vilar das Donas. I'm sure I'm missing some other good ones, too, but this is a good start. The Gitlitz/Davidson book will give you a lot of good ideas.
Dave