GR 4 and GTPAOne of my favourites in France is the GR 4. This trail traverses whole France but I like the Southernmost section in the Provence best. It starts in Grasse which you can easily reach by bus from Nice - and it is very easy to get to Nice with low-cost airlines! I therefore hiked three different times (2006, 2007 and 2016) on the GR 4 and some sections even twice!
The GR is ideal for shoulder season because it does not go up very high - but when I returned in 2016 for a third time I had bad luck. I came during Eastern holiday and in higher altitude there was still complete snow cover. But with a blue sky this is quite enjoyable ... My favourite part of the GR 4 stretches from the beginning in Grasse to Pont St. Esprit, a total of 575 kilometres.
This being the Provence you hike a lot along lavender fields but also on high mountain plateaus - or to sum it up: There is a lot of variety and beautiful views. The trail is very well marked and the first two times I even hiked without a GPS and never got lost once. There is an excellent French topoguide that covers both the GR 4 and GTPA and this book is really all you need.
What I really enjoyed about both trails are the many dry stone terraces and the abandoned villages in the middle of nowhere. Most of them would be the perfect setting for a movie! But not only are they pittoresque but they make free camping extremely easy because where there is a terrace there is flat ground! I still want to mention that wild camping is mostly illegal in France but practically I never had a problem - and there is no one else around anyways ...
GR 70 or Stevenson Trail
To sum it up: You cannot go totally wrong on the GR 70 - it is a pleasant and easy hike. But there are nicer hiking trails in France ....
GR 14 and GR 16 in Belgium: Ardennes