Bar 40 a 1
Bar 40 e 1 is a nice and cosy space with very friendly staff. It is right in the "gay corner" - the area of Bairro Alto where the gay crowd concentrates. If you go before 11 - 12am you will be able to seat and chill. After 12am, specially during Friday and Saturday nights, this whole area of Bairro gets very crowded. Therefore, the thing to do is to go in to get your drink and then walk outside and enjoy your cocktail or beer right in the street. It's what all the locals do and it allows you to watch everyone passing by whilst enjoying your drink outside.
During busy nights the whole Bairro Alto area has a Mardi-gras feel to it, given the amount of people drinking and having fun in the streets. It is definitely something to try in Lisbon. People hang around through Bairro before heading out to clubs, around 2-3am.