Rome City tour

Just check with the guide as they are good at suggesting activities for the off time. Since the hotel is near the Colosseo, you could walk there and about double the distance to the the Trevi fountain or just stroll the pedestrian streets and enjoy a dessert at one of the sidewalk cafes.

Mvsperry, most attractions are going to be closed in the evenings but the historic center of Rome is a wonderful (and safe!) city just to walk when many of the landmarks and piazzas are lit up after dark. You don't mention what month your tour will be? That can make a difference where hours of daylight are concerned as well as some later-night seasonal openings. For instance, this tour might be interesting if you're traveling during months it is offered:

https://www.walksofitaly.com/rome-tours/rome-catacombs-night-tour/

Or this:
http://www.viaggioneifori.it/en/

But pay close attention to the RS tour activity level listed for any given day? If you've spent a really long day on your feet, just finding a place to sit down, enjoy an adult beverage and do some people watching might be the ticket. A leisurely stroll works as well. We've personally never needed an after-dark plan as our days start so early; it's usually all we can do to stay upright until bedtime!