Hello coolspot, I have stayed in Lisbon 4 times and this is a good place to start with, or you could try Porto, this is in the north so this time of year it could be wet and cold, so I would plum for Lisbon. There are plenty of reviews on it on this site, so have a look at them to see what you think.
Have a look on Easyjet flight and hotels or Expedia.co.uk taxi from the airport to the centre is about 15 euros but agree a price first (as with all taxi's ) Food is very good and cheap if you eat local, and some great sites to see. As always if you do go here get a good guide book.

Dave.

I agree that Lisbon is a good place for a long weekend.There is plenty to see, the metro, trams and buses are excellent value at €3,70 for day pass as the city is very hilly. A good guidebook and a bit of planning is a good idea. There is bus every 20 minutes from the airport into the city for €3,50.