I would strongly recommend Carlos and Helena's apartment for a short or relatively longer stay in Porto. for two people it is just perfect. The apartment is brand new, and in a very central location (two steps away from the commercial part of Santa Catarina Street).
Helena was super approachable and responsive and made sure we had all the information we needed. The apartment is super clean and full of little details that really make you feel like they care about giving you a great experience and comfort. It has a fully equipped kitchen and all the regular amenities.
On top of it, upon arrival they provided us with maps and great tips about the surroundings, which if its your first time in town might really be helpful.