Just thinking you may find taking public transport much easier than driving and finding a place to keep a car in Lisbon. There are busses and trains you could consider unless you need a car aft your stay in Lisbon.

I agree with the above about how much more convenient it would be for you to take the train and skip the hassles of driving and parking in either of those cities. By the time you drive out of Porto, you could be halfway to Lisbon, and the train will drop you off in the center of town. You can rent a car in Lisbon if you need one.

If you insist on driving, then the University Library in Coimbra is an extraordinary world treasure.

As for what to do in Lisbon, any guidebook will tell you and you should pick what interests you.

If driving, you could take the www.viamichelin.com recommendation for the scenic route, but it would take 8 hours of driving time. If using a more direct route, I would not recommend Coimbra if driving, but rather a small detour to Batalha which has easier access than Coimbra.

http://www.flickr.com/photos/mksfca/4497426644/in/set-72157623788386374

Batalha - famous monastery is right on the way as is Combra, one of the nicest cities in Portugal - especially the older upper town are two spots and nearer to Lisbon Obidos to me is one of the finest walled cities in Europe - small enough so a few hours there would be great.

The must-see between Porto and Lisbon is the wonderful medieval village of Obidos. I'd also stop in Sintra although that's an easy daytrip from Lisbon. However, since you'll only be in Lisbon for two days, you may want to stop in Sintra after Porto.

Michael,
I love your photos. How long did you spend in Portugal? I'd love to combine a trip to Andalucia with a trip to Portugal. Would you happen to know the best mode of transportation from Seville to Lisbon?
Thanks.