Can rain a fair bit in December so be ready for that but can be 20 degrees and sunny. If you get the weather right sintra is really nice. Is 40 minute train from the centre.
Belem is away from the crowds a bit and has nice places down the river. 10 minute train ride from cais do sodre. LX factory is nice for a walk around but swerve drinks or food there. Is overpriced and terrible service.
If you with family, barrio alto is best avoided in evening. If you’re partying it’s where you’d go although is a bit much for me unless I’m in the right mood. Time out is pretty much guaranteed good food but can be rammed at weekends.
There are a lot of wanky bars popping up that have an almost faux hipster thing going on. Avoid anything like this as they’re weird and overpriced.
Depending where you’re staying I know a few good restaurants. Really like Picanhas in Santos but it’s not for everyone as is very basic and is pointless if you’re veggie. In fact veggie options are pretty poor all round I think. Alfama is nice for casually walking around. There’ll be lots of roast chestnuts being sold by street vendors and plenty of ginjinha.
Do not buy any street drugs.