Mejillones is located on a spur off of the main north-south highway in Chile. It's under an hour drive from Antofagasta. Frequent buses also make the trip.
The actual town of Mejillones is easily walkable. The beaches, however, are located to the north of town and require some sort of transport.
Mejillones isn't as nice as its beach-side location might imply. The town is centered around the port, and there's an industrial element to the area that isn't particularly attractive to visitors. Nicer beaches are located to the north of town, including the massive and empty Playa Grande.
There are several seafood restaurants located near the fish market, all with daily fish menus for around 6,000 to 8,000 pesos.
Several bars are in town, but the scene is dominated by the import/export crowd.
Options are limited. It's probably better to visit as part of a daytrip from Antofagasta or as part of a drive up the coast.
The next major town to the north is Tocopilla, 135 kilometers to the north. The drive takes around 90 minutes.