The Temple of Hadrian was dedicated in 145 AD by Antoninus Pius in honour of the deified emperor Hadrian. What survives today is a row of massive Corinthian columns built into a later palace, a very Roman layering of ancient temple and modern city.
The building later served as the Papal Customs House and now houses the stock exchange. It is a quick stop near the Pantheon and Sant'Ignazio, but the surviving columns are powerful enough to justify the detour.