The "big" site we wanted to see in Rome was Ostia Antica, but it is closed on Mondays, so we took the early train from Lucca in order to reach Rome with enough time to enjoy Ostia Antica.

We were catching the 8:31 am train to Florence, so we went down to breakfast around 7:15 am. The pastries at Hotel Rex were so good!
We took the 8:31 am train to Florence (for 7.30 euros [$8]), walked around for a few minutes there, bought some water, then caught the 10:38 am high-speed train to Rome Termini (for 43 euros [$47]), arriving at 12:10 pm.  Once in Rome, we bought 48-hour transit passes (for 12.50 euros [$13.75]) and made our way by subway to the Barberini stop.  There, we hiked uphill to our hotel: Hotel Modigliani.  We were able to check in on arrival!  Our room was small but just what we needed, and it had amazing air-conditioning!  I paid 88.20 euros ($97) + tax for the room.
The bathroom had a large shower and plenty of towels! After a siesta in our room, we ate lunch at McDonald's near the Spanish Steps (for 15.40 euros [$17], including the best strawberry milkshakes for 1 euro!) and then headed out, taking the subway from Piazza di Spagna to Termini and then to Piramide, where we changed to the Roma-Lido train (an above-ground train that goes to the beach) out to Ostia Antica.  We got to Ostia Antica just after 4 pm, paid our entrance fee (10 euros each [$11] + 2 euros for a map), and headed out to enjoy.  Right away, it felt a lot like the Appian Way but with more to see.
There is a blockbuster sight not far from Ostia Antica's entrance: the large black-and-white mosaics of the Baths of Neptune.  I love ancient mosaics, and this one is huge! Next, we explored the Square of the Guilds, where each store/shop had a mosaic entrance.
Another nice Square of the Guilds mosaic.  This one was very Dr. Seuss-esque. One more great Square of the Guilds mosaic!
Ostia's Roman theater was built in the 1st century AD and seated 4000! We loved Ostia Antica!
The stairs of the Capitolium were steep! I watch Rick Steves' Europe every Saturday at 6 pm on WGTE TV 30 in Toledo and have for 20+ years.  It is always fun to find myself at one of the places I have seen Rick go on the show.  This place, the thermopolium, is one such place.  It was a restaurant.  It is so well preserved!
Behind the counter at the thermopolium. These jars/pots buried in the ground intrigue me so!  We saw them in Pompeii, and then I saw them again in Louisiana.
With streets like this, Ostia Antica really reminded us of Herculaneum. The only multicolor mosaic we saw was this one on a wall.  We couldn't get closer because of the "Do Not Enter" sign at the left.
   
Ostia Antica is open until 7:15 pm, so we timed our stay to leave at about that time (and so did a lot of other people).  It appears only two trains come per hour going back to Rome, and it was a super-hot August Sunday evening where everyone was coming back from the beach, too, on the same train.  When the train finally came, it was already completely full, so my mother and I hopped on separate train cars, squeezed ourselves in sardine-like, and stood for the 30-minute journey back to the Piramide subway stop.  We then took the subway back, enjoyed evening walking in Rome (as we always do), ate dinner at McDonald's again (only 12.90 euros [$14.20] because they gave us a coupon at lunch!), and headed back to the hotel.  It was an amazing day!