I have never been to that part of the tour but i have seen the tour in the alps and I highly suggest getting up in the areas where they will be climbing. the riders will go by much slower and usually in small bite-sized groups. I would find a spot that offered a view of the riders coming from a ways off if possible. One year we stayed near the Alp d'Huez and were able to scout out a decent spot the day before by riding up it then on the day of we got up early and spent all day in our chosen spot. Take some food and water and enjoy the atmosphere.
If you speak french a radio can be handy for updates on the riders whereabouts and leaders. Also one of the RV's will usually set up television that will draw a crowd as well.
Finally, if it were me, I would head to the finishing climb. It is likely that the racing is going to be more exciting there and the atmosphere will be more frenetic and fun during the long wait for the riders.
I don't know if any of that helps.
Whatever you do, I am sure you will love it. Count me as jealous!