Viņa might be your best bet. Easy to get to, plenty of food options, plenty of beachy options with a car, etc. There is also plenty to pass a few days' time in Santiago.
While I'd ordinarily suggest the La Serena area, you'll find yourself away from good connectivity when you go to do more interesting things. The same goes for Iquique. If you've never been north, a few days in Iquique would be interesting. Get a car and drive out to Pica for a day trip (you'll have to suffer with spotty connectivity on this day trip), which will give you a taste of the driest desert in the world. Lodging in IQQ should still be reasonable as peak season doesn't start until the year's end. I haven't been up there in several years but you'll expect so-so "resorts" near the sea, and a neat old (restored) city center. Last minute air might be a bundle, however.