It's easy to get to many of the places on the tourist agenda by train or bus from Lisbon and it's not expensive. Sintra, Cascais, Queluz etc are on suburban rail lines with trains every 20/30 minutes, Mafra is easy by bus. Slightly further afield places like Évora and Obidos are reached by (long distance bus) motorcoach and as these are less frequent some planning is required.
If you plan to see somewhere on the way between Lisbon and Porto there are places such as Coimbra on the main railway line between the two cities with a train about once each hour. However between the rail line and the coast itineraries including eg Alcobaça, Batalha etc are a little awkward by bus/motorcoach because of limited schedules and a car might be better especially if time is short.
Most car hire companies do not charge an extra fee for a one way trip as it were.
Porto is similarly situated and has an excellent suburban rail, bus and tram network within the city and suburbs. If you plan to visit eg Braga or Guimarães there are again good suburban rail services. The Douro valley is tempting and again trains are a good option, especially as the line runs alongside the river for about an hour but the journey is longer.
I recommend you familarise yourselves with the timetables etc at these websites
www.cp.pt - national rail network including suburban and local services around Lisbon and Porto. The site is available in English if you click the little flag icon at the top of the page next to where it says 'Acesso Rapido'.
Timetables can be downloaded from the 'Timetables and Prices' tag on the left hand side. These are in English and Portuguese.
www.metrolisboa.pt - Lisbon metro (subway)
www.carris.pt - Lisbon city bus and tram website
www.rede-expressos.pt national long distance bus network. No downloadable timetables but, with a map of Portugal in the other hand, you can easily find point to point timetables on a possible itinerary.
www.metrodoporto.pt - Porto light rail/metro network.
www.stcp.pt - Porto city bus website which I always find hard to navigate!
There are numerous bus operations covering the areas between Lisbon and Porto. Most of the websites are only in Portuguese and some are of pretty poor quality. We made a list of those we know about at
I hope that helps as a starter.