Keep an eye out at Alcazaba Zoco — you're sure to come across some treasure or trinket among the stalls here. After you've admired the wares, try some fare from one of the food vendors.
You'll be spoiled for sightseeing choice when visiting San Javier. Alcazaba Zoco is a great place to begin here, but there are loads of other interesting sights to discover. La Manga Club, Mar Menor, and Calblanque Beach are near the community marketplace and will fill in your day nicely.