Originally built in 1880, the Alte Oper (Old Opera) was nearly completely destroyed during the bombing of Frankfurt in 1944 when the allied forces dropped over 12,000 tons of bombs on the city. After slowly being rebuilt in the 70s, it finally opened again in 1981. The Frankfurt Opera is in a nearby, modern building, but the Alte Oper lives on as a concert hall with its own entertainment lineup.
Frankfurt is relatively small, and quite easy to navigate by subway. Spending just a few hours of non-stop walking, pausing only to take photos, I managed to see pretty much all I could have hoped to see. While always nice to explore a new city, it's not exactly a must-visit, but if you're passing through anyways, a day should be plenty of time to see the sights.
Opernplatz, the plaza in front of the old Frankfurt Opera House.