I walked the Frances from mid Sept to the end of Oct 2023. My plan was that I would always take the longest route if there were options. This didn't always happen, but almost always the alternative path was a great diversion and when I joined back up with my camino family, they weren't happy with their route. Usually the alternative routes are longer but the benefit is they take you away from roads and into more nature.
I'm going to use Gronze maps to explain where these are. I primarily used the Buen Camino app while walking, and Gronze sometimes implies the alternatives are the main path...
SJPDP to Roncesvalles - the Napoleon vs Road route is described a lot. However on the Napoleon at the start of the decent down to Roncesvalles you can go directly down on the path through the forest or take the winding gravel road to the right. The pilgrims' office in SJPDP told everyone to take this road due to the conditions, almost no one did and they had a hard time with the steepness and mud. The road has amazing views, including that of the main albergue in Roncesvalles. It's far safer, less steep, but it is somewhat exposed. Towards the end there is a km or two next to a river and the road, but overall highly recommended. Extra distance - about 500m. link to map
Estella to Los Arcos - There is a shortcut "variant" just outside Ayegui, don't take this as you'll miss the famous Irache winery (free wine). Many people did though, by accident. Then, a little ways past the winery, there is an alternative path to the south, "variant de Montejurra". I don't think this path is much longer, might even be shorter, but it's far more scenic from what I heard. You get on to single track through trees which is really nice especially on a hot day. There are some terrific views and you're far away from the main roads. link to map
Logroño to Nájera - This is a long days walk, but there is the little town of Ventosa which can be bypassed saving you 1km. However this "detour" takes you along an "art walk", away from the road, and Ventosa is a nice place to stop for a drink/ice cream/tapa. The bypass is marked on the map as "camino directo". link to map
Entering Burgos - This one is essential. The main camino route takes you north of the airport, through a lot of industrial areas etc. Or, the alternative takes you south and you enter Burgos along its river. It's a stunning walk, and if the sun is out you'll see all the locals hanging out and enjoying their main park. We even had a quick swim in the river! It's about 1km longer, but that river walk is so nice and shady. There is a sign to show you where to turn, but many still missed it. link to map
Fromista to Carrión de los Condes - Just as you leave Población de Campos the Way splits and there is a northern path that takes you away from being next to the road for about 10km. Well worth it. Instead you are on a single track path next to a small river, and farm roads. You can actually see the road to you left a few hundred meters away!. Adds about 1km, but absolutely worth it. You have to look for this, there isn't obvious signs. link to map
León to Hospital de Órbigo - The official camino path takes you alongside a main road all day here, and from what people have told me, it was the worst day on the entire camino. The alternative route breaks off into the countryside just as you finally leave the outskirts of León. There are a number of small villages with albergues along this route so you don't have to make it all the way to Hospital de Órbigo. It's mostly still on roads, some dirt, but there were almost no cars. We stayed in a casa rural called Molino Galochas (just after Villavante), which I highly recommend, book though. This detour is about 4km longer. Gronze documents it over two stages. link to map
Hospital de Órbigo to Astorga - As you exit Hospital de Órbigo there is yellow paint all over the road telling you to turn right. Most people missed it, don't! Gronze suggests it as the main route. Again you avoid the main road, and there is the best snack oasis of the camino on this path, plus decent views of Astorga. <1km longer perhaps. link to map
Entering Ponferrada - many people stick to the side of the road going into town, but the Way breaks off to the left through some more countryside. It's a tough decision because it's a hard day of walking and this adds a bit more of a hill and about 1km. Nothing amazing to report about this section but you avoid the main road ("variant por carretera"). link to map
Entering Villafranca - another must do.... just after Pieros at the top of the hill, you turn right away from the main road, as per Gronze's suggested path. This takes you through some very scenic countryside, and away from the main road, adding about 1km. link to map
Leaving Villafranca - I didn't do this alternative as guides and people suggested not to. It takes you up a mountain which is fairly steep and it was pouring down the day I walked this. But it's on the Gronze map. link to map
Triacastela to Sarria - well documented southern route to the monastery in Samos. About 7km longer but well worth stopping there and doing the tour. A really beautiful trail that day too, but I'm sure the other way is great too. link to map
Leaving Portomarín - this one isn't on Gronze's maps, but just as you cross the bridge leaving town you can go right following the road, or left following a lesser used road. I went left (longer), it was nice - very green, no cars at all. There are two camino markers pointing in each direction at the end of the bridge so you can't miss it. Probably <1km longer and you get views of the river for a bit.
There will be other detours and alternatives that you see on your Camino. I encourage you to explore..... Buen Camino!
I did the dragonte route leaving villafranca, and it was absolutely amazing. Definitely challenging, but we’ll worth it for the views and lack of pilgrims. Highly recommend!