Hi!
You best bet for parking in Lisbon coming from the North is to head towards Gare Oriente Rail/Bus Station/Vasco da Gama Centre. This is near the airport bordering the Tejo and has the advantage as being linked to the Metro (Linha Vermelha), and the suburban rail network.
From there you can get the Metro to Alameida Station and there change to the Linha Verde from where you can get easily to Rossio/Baixa Chiado for the centre of the old town.
Any way you come definitely avoid driving into Lisbon. Park and use the Metro/Suburban Rail Services.
You can find the metro site in English here:
http://www.metrolisboa.pt/Default.aspx?tabid=271
As regards Porto, I do not know the city as well as Lisbon and the South. However, there are plenty of reports that traffic there is bad as well.
If driving I would suggest that you head for the F.C. Porto (Estádio do Dragão ) football stadium and find parking near there. Thereafter you can use the the Light Metro System to travel around.
See the following reference:
http://en.wikipedia.org/wiki/Porto_Metro
Another option as regards Porto is to take the train.
According to the current CP timetable, there is a morning departure as follows:
Leiria 7h10 Porto Campanha 9h44
(Change in Coimbra)
22 Euros one way 2nd Class
There is a return at:
Porto 17h45 Leiria 20h21
I think this would be an option to consider as it only takes 2hrs 36 mins, would avoid paying motorway tolls, take you the centre of Porto and avoid a lot of stress.
Hope this helps.
Check out my blog at
http://barlaventostravelblog.blogspot.com/
where you may find further info of interest.
