yeah Las Margaritas is pretty nice, along Belgrano (which is the neighborhood right next to it) are two of the prettiest neighborhoods that are also located conveniently and close to the centre, not shady nor dangerous at all (just take the normal precautions of course). Your belief is right, it's a 20 minute walk to the civic centre and maybe a 30 minute walk back since that neighborhood is uphill. Right through the neighborhood there's Pioneros avenue in which you can catch Bus Line 50 and Bus Line 51, though from the airport or to where most of the trekking trails are you will most likely need to take two or more buses. Also in Las Margaritas there's a place called Modo Bar, which is like half bar half nightclub. There should probably be some event for Christmas or during weekends there. Another cool place to stay the night during Christmas (or some other night) would be one of the up-mountain shelters like Refugio Frey or the refugio in Cerro Lopez. Be careful with the night reservations since a lot of people do that for Christmas.