Please note I am not a travel agent! I am happy to answer specific queries and provide general comment.
First of all, August is peak tourism season here in the Iberian Peninsula. It is also very very hot. This summer has been hotter than usual. Consequently, I would strongly not recommend heading into the following regions of Spain as you will not be able to do anything in the daytime. - Extramadura, Andalucia, Castilla y Leon. The Asturias and Galicia should be ok owing to the influence of the Atlantic. I would also have to say that I cannot recommend visiting the interior of Portugal during this time as temperatures also can hit between 35ºC and 40ºC during the day.
I would suggest that if you want to visit the North, you consider flying from Frankfurt-Main to Porto in the first instance. Pick up a hire car there and visit Guimarães, Braga, Ponte de Lima, and then swing into Galicia, Spain and possibly visit Santiago da Compostela and Ourense. Head back in to Portugal and visit Chaves and Bragança. From Bragança head south towards Viseu and Coimbra. from Coimbra head towards the Algarve bypassing Lisbon. Head for the Barlavento region of the Algarve around Lagos and Sagres which is slightly cooler owing to sea breezes.
The head back to Lisbon by coast roads. Dump the hire car and visit Lisbon using public transport which is much easier.Then head up to Porto again by train or bus.
For distances between towns I suggest you use Google Maps or viamichelin.com to calculate these.
A final point, as regards car hire, remember to specify automatic shift (which will atrract a premium) as most vehicles this side of the Pond are manual shift. I also recommend a vehicle with air conditioning for the time of year in question.
