Buy or take a book (English books are to the front right as you go down the internal escalator)!
Buy a coffee and sit in the cafe and read.
There are usually a lot of interesting people there and you might well strike up a conversation. Most young Lisboetas speak English.
Also go and have a coffee at Kaffehaus, also in Chiado. Can't buy a book there but you will probably strike up some conversations.
Another similiar place, albeit a bit more expensive is Cafe Fabulas
