I traveled the length of Chile earlier this year and visited San Pedro de Atacama, Antofagasta, and La Serena in Norte Chile.
San Pedro is a nice little oasis town which is increasingly popular with travelers. I pit-stopped in San Pedro after an amazing 4 day 4WD trip across the Bolivian southern altiplano from Uyuni, Bolivia. From San Pedro, the popular attractions are the Valle de la Luna (there are a lot of Valle de la Lunas in South America), the El Tatio geysers, and Laguna Chaxa for the flamingos. But the number one reason for the flock of travelers to this tiny town is to do the 4WD trip mentioned above. If you're up for a little adventure and willing to trade comfort for some spectacular scenery, I highly recommend it. You may want to note that San Pedro is at about 2500m/8200feet above sea level (The El tatio geysers are over 4000m/13100feet), and some may be susceptible to altitude sickness.
AFAIK, there are no flights to San Pedro. The nearest airport is in Calama, about an hour's away from San Pedro.
I didn't find anything interesting in Antofagasta. It's the biggest city in Northern Chile (and supposedly the second biggest Chilean city after the capital Santiago) and so there's the usual city scene.
La Serena has a beautiful beach, although as I was there in April, it was too cold to take a dip. The beach was deserted then. But when I walk along the endless stretch of paved walkway by the beach, passing by blocks after blocks of motels, condos, cabanas, and hotels, it's a sure bet the beach is a major Chilean summer destination.
From La Serena, there are daytrips to the surrounds, such as the Elqui Valley, where there are a few wineries for the Chilean wine fanatics, or maybe some pisco sour tasting!
I also did a night trip to the Mamalluca observatory. Because the skies over La Serena are very clear throughout the year, there are quite a few 'professional' observatories located in the area. While some of them are open during the day for a poke-around (with advance notice I believe), none are open to amateurs at night. So the enterprising town of Vicuna (45mins away from La Serena) plonked some cash and set up a pretty decent observatory. I'm not an astronomy fan by any means, but Mr. Pumpikin is, so we went (along with like maybe 50 other tourists). I was glad we did though. The staff did a presentation about astronomy, led us outside and showed the constellations, and then brought us up to the observatory, where through the telescope, I got to see very clearly the moon (and all its craters and bumps) as well as saturn and its ring. It was quite enjoyable. Of course, there was the requisite gift shop attached to the observatory too. Many tour agencies in town run trips to the Elqui Valley as well as to the Mamalluca observatory.
Chile is just beautiful, and the Chileans are so hospitable and gracious. It's my favorite country in South America and I would love to return one of these days.
Happy travels!