I will echo the sentiments that most of the cities mentioned by the first poster are too far away for day trips. Alcobaça, Batalha, Foz, Peniche, Tomar and, of course Coimbrã are much closer. I will also add Conimbriga, close to Coimbrã. It’s the ruins of a Roman town and still has a lot of lovely mosaics. It’s very much worth the time to visit.
There is also a wolf sanctuary close to Fatima that might not be too far. Sorry but I can’t remember the name. But, it’s a worthwhile, off the beaten path, visit.
FYI - driving in that area is also easy.