Coimbra has been long on my list to visit especially because of the library. Oh was it worth it. I was actually really surprised and I'm not quickly surprised anymore. The St. Michaels Chapel is also a piece of art and I was lucky to see some kind of celebration there. Not sure what it was actually. The Santa Cruz Monastery was also very nice but at first i didn't realize there was a door to the cloister behind it. A bit hidden.
Of course: everywhere tiles! They are beautiful. I visited when I drove from Porto to Lisbon. It's easily visited, a long day but it's easy to do. From Porto it makes a nice day trip too, from Lisbon it's a bit far away maybe.