Sus consejos:Go visit Belém, and Sintra too if you have time. Start your night out at Bairro Alto, drink Amarguinha, an amazing almond liquor that you drink with lemon and ice, eat Pasteis de Belém, check the view on top of Parque Eduardo VII, go to the beach, get lost in Alfama, go out at LUX, bring good shoes; the city is built on 7 hills. The city is safe but don't be too naive in Bairro Alto and watch your bag a bit, most sightseeing spots in the city you can do by foot, but otherwise the metro is really convenient and modern.
I lived in Lisbon for 8 months and fell in love with the city. It has such a friendly atmosphere, almost always sunshine, prices are still reasonable, beaches are nearby, beautiful views, and I can go on like this for quite some time. Perfect location for a short city-trip, since you're there in an instant, or a longer holiday because it can be your starting point for a trip through Portugal.