Take a sightseeing or dinner cruise on the Tevere (Tiber River) in Rome
After more than a century of pretty much ignoring the Tiber (building high embankments seems to doom all urban rivers to such mental neglect), Rome now has a river cruising service like those in Paris and London.
Not that is it anywhere near as good as those in Paris and London—the tall 19th cenury embankments flanking the Tiber in Rome hide much of the city from view, and the commentary on the day cruise is pretty canned.
However, it does make for a rather different take on the Eternal City, and a bit of a relaxing hour away from the relentless sightseeing of ancient ruins and Old Master art decorating the countless churches and museums.
The new rubber raft tour is intriguingly original as well.
There's also a 24-hour hop-on/hop-off boat serivce up and down the Tiber River (Fiume Tevere) in a one-hour loop accompanied by recorded info on what you are passing blaring in 7 languages.
The loop runs from Isola Tiburtina (Tiber Island) to the Ponte Duca d'Aosta bridge. (You can also combine this with a 48-hour hop-on/hop-off double-decker bus tour ticket—best of both.)
The main embarkation point for the cruises is Ponte Sant'Angelo, in the middle of the route, across the river from Castel Sant'Angelo, just a few yards upriver from the bridge itself—more specifically, along the Lungotevere Tor di Nona between where Via del Mastro and Via della Rondinella hit it.