Trajan's Market is across the Via dei Fori Imperiali from the Forum, and from the street looks more or less like the forum - ruins, some reconstructed, some falling down. During ancient times, Trajan's market housed a variety of shops, most of which are today surprisingly well preserved.
Unlike the sites across the street, you have to pay to get into the market, but that also means it's usually relatively empty, so you can have the run of the place. Plus the preservation of the market means you’re walking through actual buildings, and you have less to reconstruct mentally for yourself. If you're only in town for a day or two, it's not really worth a stop, but if you're in Rome for awhile, it's an interesting spot to wander through. Plus, the near emptiness of the location makes it a nice place to take pictures.