There a number of "standard" building types a city would have including temples, stoas, theaters, gymnasia, council-houses and fortifications.
The ancient monuments provide all sorts of engineering interest, of course, I don't list the obvious ones. But here are some other ideas you might not think of.
The Rio Bridge
The Tower of the Winds in Athens
The canal at Corinth and the U.S. Corps of Engineers
The fountain of Peirene in Corinth and its lengthy tunnels
The medieval forts in Nafphlio
The theater of Epidaurus with its amazing acoustics
The Tunnel of Eupalinos on Samos