Porto to Lisbon

I would recommend stopping in Coimbra which is approximately an hour and 20 min away. The University of Coimbra with the Biblioteca Joanina is a must see..( one of the top libraries in the world)
Also we found one of the best places to buy Portuguese made ceramics (not the ones from China sold in tourist traps in Lisbon) was in Coimbra.

Thanks so much for your reply.

If you simply want transportation from Porto to Lisboa I would recommend the train. The AP (Alfa Pendular) trains are very comfortable, run regularly and take 3 hours. Promo tickets purchased in advance are about 13 euros. If you would like to sightsee on the way then obviously you should rent a car and I agree that Coimbra is worth a visit. Possibly Obidos as well and it is on the way. To rent in Porto I would use web sights like Expedia or carrentals.com to shop for car rentals. You will not want a car while in Porto or Lisboa. Yes, Porto is the same as Oporto.

Thanks Scott

We have rented a car to drive from Porto to Lisbon. We are thinking of spending one night along the way to see the towns along the way. What town do you suggest to spend the night?

We have rented a car to drive from Porto to Lisbon. We are thinking of spending one night along the way to see the towns along the way. What town do you suggest to spend the night?

For just one night, you could stay in Coimbra, and dispense from renting a car. Trains are fast and cheap, and Coimbra alone will easily fill the afternoon and following morning.

Thanks Balso. We want to see more of the little towns along the way so a car will provide us with the flexibility. We did book a hotel in Coimbra!

Ok.
There aren't many worthwhile diversions between Porto and Coimbra in my opinion, so I'd focus on getting to Coimbra early enough to visit Coimbra.
You have more options for the next day: on your way to Lisbon, you could swing by Alcobaça or Batalha, then Obidos, for example.