Jardim Alfredo Keil is a small and lovely park. There are a lot of benches to have a rest and drink coffee. In the middle of the park is a cute fountain which offers a nice atmosphere. It's easy to reach directly from Avenida de Liberdade but the park itself is still really quiet.
The park is easy to find and reach if you are in central Lisbon. Also if you need a break from shopping and the noisy atmosphere of Avenida de liberadade it's a perfect place. You will meet some locals having lunch break there and also meet the guy who lives in the park. Don't be scared!! :)
There is a cafe called "Brooklyn" where I always buy my espresso for 65 cents if I have some time to enjoy it.