Driving and parking is an awful hassle in and around Lisbon - and in any Portuguese city. If your parents have mobility difficulties, then Sintra isn't the best idea: even with a taxi or car there are slopes, steps and lots of walking. Evora's not easy for navigating and quite a hike for a novice day trip. It might be easier to take a trip by train to the beach resort and dormitory suburb of Cascais. Trains take 30+minutes and run every 20 minutes or so. Lunch at some beach cafe or somewhere smart (like the Hotel Albatross) overlooking the sea.
Unless you have ties with the place, I don't really get Aveiro as a base. It's probably easier all round to take the train to Coimbra, overnight there, then resume the train journey the next day to Porto.
Of course, a car can be a great convenience, and there are delightful places on the coast - and inland - as you move north from Lisbon. Perhaps you are trying to pack too much into a week, though i understand the urge to see the lot...