You don't need one - just get a 90 day tourist visa at the border.

Hi - The biggest factor that will effect the route you take will be the amount of rain that is / has fallen at this time. Some roads will be closed at this time, and although you may be able to get through via another route, there may be longer journeys invloved than in dry season.
The easiest and most direct route would be something like La Paz - Sucre - Potosi - Utyuni - Tupiza. However this is missing out on the lower jungle areas, which have some of the best wildlife viewing in the Amazon basin. As you only have 6 to 7 days I think you rmihgt be stretching it a bit to cover both the Jungle and the Altiplano area in this amount of time. So you should pick one. Also bear in mind that trips out to Rurrenebaque by plane don't guarantee a return date in wet season as the planes are often delayed as they can't land or take off due to weather or rain. I have been stuck in Rurrenebaque on numerous occasions. Although it's not a bad place to be stuck!

Assuming you decide to go on the Altiplano route outlined above, I would recommend seeing / doing the following things if nothing else:
La Paz - Death Road Mountain biking - GREAT fun!
Potosi - Mine tour
Uyuni - Salt Flats (Do not miss this!) It will more than likely be covered in Water in Jan, but it's still beautiful. Try to get your jeep driver / company to go at sunrise or sunset if it is too wet too drive accross as it's a view you will never forget!
Tupiza - take a trip out of town to see the countryside - it's stunning.

From Tupiza you can make your way to Salta in a (very long) day's travel.