Just got back after 4 days in Lisbon/Sintra.
Top tip: Do your research on where everything is before you go - the geography of the city means everything is spread out in a way that makes it different to any other city.
Top site - the Cristo Rei statue - the view from the top is the best I've seen anywhere in the world. We got an Uber from the city centre over the suspension bridge for 12 euros (you can get the ferry/bus but some of my family don't have sea legs and it was a bit rough!).
Also worth seeing: Belem Tower, The Monument of the Discoveries, Jeronimos Monastery, The Pantheon, Rua Augusta Arch/Praca Commercio, San Jorge castle (more stunning views).
Sintra was an experience - gale-force winds, driving rain and fog made the Moorish Castle an atmospheric and challenging experience!
The wooden trams are overrated - great to watch and photograph, but everybody jammed in like sardines and they crawl along like an asthmatic slug. Metro is good but doesn't cover all areas so we used Uber a lot which is very cheap for the distances involved.