Hello , everyone! We are going to Portugal  only for 6 days and it’s a very impromptu trip. Can you share your suggestions of what I can cover in 6 days( Lisbon, Sintra, Porto), May be I can add something or it’s already to ambitious for such a short trip.
The plan is:
Day1 
 Arrive to Lisbon, rent a car and go straight to Porto.  Use Porto as a base for 2 or 3 nights . 
Day 2:
Douro Valley day trip from Porto , 
Question: which trip to the Valley it better: train or river ?  Is it possible to do a half day trip to a valley from Porto?
Can you recommend a tour guide or tour company?
Day 3 : spend a day in Porto and leave at night for Lisbon  or  leave Porto early next morning.
Day 4 
Leave Porto ( or Douro) early in the morning , dropping off the car at Lisbon airport  and spend the rest  of the time in Lisbon. 
Day5  Sintra. 
Day6  Lisbon. 
Day7 flight Home 
Does it all make sense logistically?
Considering that we are driving to Porto and back from Lisbon May be you can suggest any stops which are on the way. I understand, that my questions are very sporadic but  we booked tickets very spontaneously  and I feel a little overwhelmed . 
  Thanks in advance . All suggestions will be greatly appreciated. I did not book anything but tickets yet so very open to any changes in the plan.  The only place that I am not ready to drop is Sintra everything else is an open subject