Rooftop restaurant in Santiago, Chile

The Costanera Center is the best future bet for such facilities. The tower itself is still under construction. It will be 300 meters high and South Amarica's tallest building. It seems likely that it will at least have an observation platform of some kind.

Currently available are several apartment hotels with rooftop swimming pools and lounges. You would of course have to stay in those locations to take advantage of the facilities. I stayed in a hotel where the breakfast lounge was in the 17th floor. 17th floor just don't impress much in this city.

Rather on random I found a hotel named W Santiago (link to a Tripadvisor review) which looks like it has relevant facilities. I have not been there myself, but the reviews are good and pictures indicate a very nice rooftop pool. I do not know if there is a general access restaurant.

One idea that might be fun is to take the funicular rail to the top of Cerro San Cristóbal. You would probably have to bring food with you – there aren't really any restaurants at the top; just a few empanada vendors.

But there are tables and chairs set up there where you could look out over the city as you are enjoying your meal.

The funicular costs 2 lucas (2.000 pesos) for an adult round trip and 1.500 pesos for children and runs until 8 PM, or you can hike/drive up the side of the hill.

Note that you will get the best view after a rain; Santiago has a bit of a haze problem.