You've chosen an amazing country to visit. I recommend it highly. My husband and I visited in Oct/Nov '03 and fell in love with Portugal. It rained quite a bit, but it only added to the drama.
Since you mentioned wanting to visit historical places, I recommend visiting the site http://www.pousadasportugal.com/, which details pousadas throughout the country that have been made out of castles, monasteries, etc. We stayed in several while we were there, and it really adds to the experience.
I would also recommend Hotel Bucaco (or Bussaco), just north of Coimbra. It's a hotel that was originally built as a royal home. Lots of history and, if you can afford it, a wonderful building, gardens and tons of trails for walking & discovering.
We rented a car while we were there - the highway system is easy to navigate, and we did not have any problems.
Good luck -- you'll have a wonderful time!!