Hi D,
I've been to Lisbon several times now and every time I go my intentions are to travel outside of the city... Ha!
It really depends on what type of things you like to do when you travel.
Exploring Lisbon and it's back-streets is a job in itself. Wandering the Barrio Alto at night and being tempted in some little doorway by the floating music of a tango being played is just one reason why I can't leave the city when I get there.
After three or four days you may feel the need to move on and see further afield. But why go so far when you can always hop a train in the city centre that will take you to Sintra - there's another day spent.
Ok, so you've had enough of the city, but maybe you got up too late to travel to the Algarve. Well, you just hop on another train from the city and at the end of a delightful 45minute trip along the river bank and the Atlantic estuary you are in Cascais. A proper Portuguese holiday resort with all the trimmings.
I wouldn't be in any great rush to leave Lisbon but next time I go back I've 'really really' promised myself I will take a trip north along the coast to Porto... Let's see.