What are the must-sees in Porto for a full day and a morning?
In no particular order:
Wander round the Gaia side of the river and take a port lodge tour - Sandeman is good.
Nice restaurants for lunch/dinner.
Visit the main railway station - beautiful traditional Portuguese tiling.
Old Bookshop - Livraria Lello - said to have inspired JK Rowling for part of Diagon Alley.
Tram ride
Go to the Things to Do section for Porto on TripAdvisor to get ideas. My must see places are probably different than yours.
When we were there we did a lot of walking. Places to visited included Church of Souls, San Bento train station, the central market, Church of San Ildefonso, town hall with the Porto letters, both sides of the river, Rebeira pier, did port tastings, the plaza by the Church of Carmo, and a few other places.
For the tourist centric things, we took the touristy 1 hour boat ride and went to the Majestic Café. We skipped the book store because we felt the entrance fee was too pricey for what it is.
Not mentioned above but I feel would be a must see is the Palacio de la Bolsa. Unfortunately, when we were in Porto it was closed due to a private event but online photos of the interior are amazing.
I would also agree with seeing the Sao Bento train station just for the azulejos.
We also skipped the bookstore due to the price just to enter .