I'd go into town, have diner (quite a few restaurants open late), get a hotel/hostel, and wake at 5/6 am to go for a walk/ride before returning to the airport. This hostel and this one are fairly central.

Here is a list of restaurant suggestions.
I can recomment Cufra (not on the above list). location: Avenida da Boavista,number 2504. Open until 2am
I'd go for the Ameijoas 'a bulhao pato - clams - as a starter.
The very local Francesinha as a main or Secretos de Porco Preto (Black Iberian pig tender fillet)
Grilled fish if you're not into meat. Ask for the catch of the day.
A bottle of red wine or Superbock if you're into beer.

Go for a drink at Cais da Ribeira afterwards for a drink. Lots of bars and you'll see D. Luis bridge (one of the must see) by night. You mentioned no nightlife, but if you can make an exception you can skip a hotel/hostel as bars and discos can keep you going until 5/6 am.
Sleep
Wake up and walk around.
Breakfast at Majestic. Some Port wine to get you going
Check these top 10 suggestions.
I'd also visit Lello bookshop.
If you'd like to have a look at the ocean - Foz is the place
Go back to the airport. See here the metro schedule

Enjoy and don't stay at the airport! Too much to do.