Cerro Paranal is amazing. I went there last year - rented a 4wd in Antofagasta and drove 100km south to the site through the Atacama Desert. The drive itself is spectacular as well.
They run tours on the last two Saturdays every month, but book well in advance...
I can recommend staying after the tour has finished. You'll be rewarded with a fantastic sunset and of course some serious dark sky! The zodiacal light was very impressive.
Oh, and dress warm, it gets very cold at night. It's not cold during the day though.
Tours can be booked here:
http://www.eso.org/public/about-eso/...nal/index.html