Perhaps a bit of a cop-out in terms of personal recommendations, but I don't know what your interests are. Nevertheless, I can strongly recommend the Mini Rough Guide to Porto. Cheap, pocket size, and comes with a free e-book, which is handy for the Kindle. We preferred Oporto to Lisbon, as it seemed less crowded and not as touristy or as expensive. Late September is also a good time to go.
We've just come back. I can fully recommend Portobay Flores Hotel. It's five star, a wonderful mix of an uber modern superb accommodation attached to a beautiful 15th century palace. We had a wonderful and very comfortable stay there. Nothing was too much trouble for the staff. Mainly the guests were lovely Americans. It's also very central so everything is within easy walking distance.
Loads to do in Porto. Be aware, it's quite steep and hilly in places. Take in Cafe Majestic and Rua de Santa Caterina - art nouveau masterpieces. Also Livraria Lello - apparently J K Rowling was influenced by it during her time living in Porto (you do need to book online as it's time slots to visit). Lots of the old buildings are covered in majolica tiles, stunning.
There's a lovely vibe on the flat waterfront of the River Douro - with great cafes and restaurants. We booked a river cruise which was fascinating and you got a great view of the city from the water.
We ate in some superb restaurants - both fish and meat excellent. Not cheap though. For brunch/lunch the meat and cheese platters are very good.
There were plenty of little markets dotted around the place - mainly cork based stuff like notebooks etc. and lovely textiles. The higher end shops are around Rua de Santa Caterina - Zara was a much more superior version to the UK offerings. Also it was great for clothes for the grandchildren, as was Bennetton. There is also a lovely shopping mall here too - all under cover with cheaper food offerings!
It has great street music - fantastic young singers and Spanish guitar players. It felt very safe. All in all, not having been to Portugal before, but a huge fan of Europe in general (Italy, South of France, Balearics, etc) it was so much better than we anticipated. A wonderful place for a visit - we were there for four days which was enough time to fully explore. Hope you have a fab time!!!!!