Hop-on/hop-off tours are actually a good time.
I saw hop-on/hop-off double-decker tour buses in many different European cities throughout my travels, and I always kind of sneered at them. But because I only had two full days in Rome, I decided it would be the best way to maximize my time.
I got a 48-hour pass for about 26 euros, which meant I could get on and off as much as I wanted in that time period.
On the first day, the bus’ cover protected me from what was unfortunately a rainy afternoon in Rome. On the second day, a blistered foot was bothering me on my trek to Circus Maximus from the Colosseum, and the bus almost magically appeared ahead of me.
One thing that disappointed me was that the various stops weren’t as close to the landmarks as purported. But it still sped things up.