There is either a lot to be said about this odd triumphal arch, or a little to get the point across. This website (Link) does the latter quickly and succinctly.
Please see this website (Link) for 3-D restorations of what the Temple of Olympian Zeus and the Arch of Hadrian originally looked like. We see the ruins, but we must remember the ruins were once beautiful buildings or monuments, painted and decorated for the viewer to enjoy.
The first full day of walking around Athens ends at the Lysiskrates' monument at the end of the Street of the Tripods, which existed in antiquity. The street was lined with monuments like the one 
discussed at this website (Link), which honored the wealthy aristocrat (choregos) who paid for a chunk of an ancient playwright's plays to be performed. If the plays the choregos sponsored won first prize, he - the choregos, not the playwright - got to erect a monument. Lysiskrates won in 334 BC, and his monument is one of the few to survive to today. Mary Beard, one of the most distinguished scholars of ancient Greece, talks about the monument here and the video is well worth watching (Link)(4:47) to wrap up the first day with the theme Past and Present.
National Gardens and Zappeion Gardens
This website provides a quick overview of both the National Gardens and the Zappeion Gardens, which we walk through on the way to the Temple of Olympian Zeus. Be sure to return when you can and enjoy the quiet shade.
This website provides a quick overview of the Zappeion Gardens.
This Website (Link): has a quick overview on the Temple of Olympian Zeus
This quick overview video (Link) includes a two-minute video of the temple from different perspectives with views of the Acropolis and other parts of the city.
This University of Chicago website (Link) contains good background on the Temple of Olympian Zeus' construction, which underwent several phases over many centuries. Stop reading at the paragraph "Then, in AD 267, Athens was sacked by the Heruli, a Germanic tribe from the Black Sea region (Zosimus, New History, I.39) and the temple burned, as was the Painted Agora, the ruins being used to build a new inner circuit of protective walls." BTW: don't be a Herulian ...
