The best option to travel from the Porto Airport (OPO) is by using the train. From there, you can take the metro/underground (situated within the airport) to Campanhã metro station by using the E-line.
Useful information regarding travelling with the metro:
Campanhã is the main train station in Porto. Here you should get a train to Coimbra-B (main train station in Coimbra). The fastest option is Alfa Pendular but IC (Inter-Cities) lines are a good second option. Other options will be a bit slower, but will give an interesting outlook over the beautiful landscapes.
For the train timetable, pricing and purchasing tickets, use this link. The origin is Porto-Campanha and the destination is Coimbra-B.
The best option to get from Lisbon (LIS) to Coimbra is by using the train, same as with Porto. The best way to get from the airport to the Oriente train station is by using a cab, taxi or uber.
Oriente is the main train station in Lisbon. Here you should get a train to Coimbra-B (main train station in Coimbra). The fastest option is Alfa Pendular but IC (Inter-Cities) lines are a good second option. Other options will be a bit slower, but will give an interesting outlook over the beautiful landscapes.
For the train timetable, pricing and purchasing tickets, use this link. The origin is Lisboa-Oriente and the destination is Coimbra-B.
Other option is to use the airportshuttle service. They bring people from the airport to the desired hotel/residence. More info can be found on their website.
Notice that they don’t refer a specific town because they use to bring people to different destinations on the same shuttle.