If you want to stay for a week this is what I recomend:
Stay in La Parva (easy conection to Valle Nevado) in the "condominios" (Small flats). La Parva is a small but with good off piste ski centre and have no hotels cause it´s intended to be for wealthy chileans that go every weekend to their flats.
Another good thing about la parva is that you have the best night life of any southamerican centre and is a 5 minutes car ride to Farellones and "el colorado" which is another centre with an even better nightclub.
La Parva, Valle Nevado and Colorado are 1 hour from Santiago.
Portillo is around 2 hours from Santiago but on a different direction than the other three.
From La Parva you can ski to Valle Nevado (a lot more tourist directed) or by car you take around 30 minutes.
Portillo is a good choice also. I think you can get more snow here than on the other three, but you won´t have many other choices to go to from portillo.
Bottom line stay where ever you want (except colorado) and you´ll have some of the best skiing you´ve ever had. Nice people, excelent snow quality and the best weather.
If you have time for doing some travelling go south to Patagonia to the "Torres del paine National park" or go north (dessert) to "San Pedro de Atacama" really two amazing places worth a visit.
If you surf or windsurf you´ll get world class waves and wind in the coast but thats a whole different story. Cold water anyway.
Good luck and see you there.