The Coffee One in Pontcanna has huge windows, is spacious, and seems to attract quite a number of people with your idea. And Mrs SR has just mentioned the cafe inside Cardiff Castle. It has some serious windows, and looks across the Castle grounds. Finally, there is a coffee shop in St David’s 2, there is a coffee shop on the second level on the left-hand side (when you’re facing the castle) that overlooks the Hayes.
Oh, and two more suggestions: there are numerous cafes in Penarth, and of course, there is the cafe on the ground floor of the Millennium Centre across from the free stage and beside the souvenir shop. That one is actually quite peaceful, and proffers a great view of the Basin.