Under the famous (and famously crowded) Trevi Fountain is Vicus Caprarius (AKA City of Water) archaeological site. This site is part of the aquaduct that served the water flowing from the Trevi Fountain, as well as many ancient homes in the Trevi district.
It’s essentially located under Harry’s Bar Hotel, right by the Trevi Fountain. You can actually access it (with a reservation) right from the hotel. We didn’t have a reservation but there was one in an hour, so we took that spot and headed into Harry’s for a drink while we waited.
At 4, we headed over to the site and checked in. They offer audio guides, as well as scheduled guided tours. We didn’t do either but the signs did a good job of explaining what you were seeing.
At only 4€ and about 20 minutes total to visit, it’s definitely a great way to take a break from the more crowded attractions in the area. Reservations are required and can be made through the website or WhatsApp, or by stopping at the site.