On the way I plan to visit Obidos, Batalha, Convent of Christ and the Roman Ruins that's mentioned above as time allows.
I thought about splitting the stay: one night in Obidos or Tomar and one night in Coimbra. That'll eliminate any backtracking. But these towns are so close together, and as mentioned, in winter there may not be as much to do in the smaller towns, I figure may as well stay in Coimbra, with a plan like this:
Day 1 - Leave Lisbon, visit Obidos and Batalha (skip Batalha if no time), continue driving to Coimbra; walk around Coimbra in the evening, stay in Coimbra
Day 2 - Back track to Tomar, visit the Convento and visit Roman Ruins; walk around Coimbra in the evening, stay in Coimbra
Day 3 - Drive to and tour Aveiro; stop by Castle of Santa Maria da Feira; Arrive in Porto by night