I am not an expert flyer but that doesn't sound like a ton of time ... Coliseum is cool but is best with a tour, if you can squeeze that in its a good call. I don't think you have enough time for vatican/sistine chapel... If you want you can keep it simple - walk around rome there are probably some free audio walking tour apps, and see a cpl things like the trevi fountain, spanish steps, pantheon and just general culture. Maybe uber over to Baghetto dairy for a bite.