When are you planning on being there? (Winter?) If it was me, I'd probably head right to the Mercado Central and have a nice lunch (taxi from SCL) The rest depends on the weather and day length. I'd then likely take a short walk up to La Moneda and then take the Metro up to the Pueblito de los Dominicos (which is a long metro ride - last stop on one of the lines) and walk around there - lots of interesting handicrafts. Last time I was there it was raining lightly and not too bad. Depending on time (and weather) - if inclement I'd head to one of the malls out that way (Alto Las Condes or Parque Arauco - yes, I said it, but if the weather is crap, I'd go there, too) and if not, probably head back to Providencia or Las Condes, walk around a bit, and then take a taxi back to SCL. I'd recommend being @ SCL at 7:00 PM.

Your noon immigration line should not be bad. There are plenty of other things you could do, like if the day is really clear, after lunch head right over to the funicular in Bellavista and take the ride up. If you wanted to make your meal later, you could take a taxi right there first. If it is a crystal clear day, it is worth it. The rest of Bellavista doesn't have much going on until nighttime, but there are a lot of lapislazuli shops in that neighborhood if that is your thing.

The evening departure migration lines can get bad. If you happen to be in a premium LAN class, there's a little special line followed by a no-wait security line just next to the premium check-in at the far end of the terminal. If not, enjoy the masses