I have been on 12 RS tours. I don’t do a lot of planning in advance. Here’s what I do to prepare:
I read through the itinerary and make a list of sights that are specifically mentioned. I also assume that “walking tours” will include most important sights. Then I make a list of other sights that I think I might want to see or visit if there is time. I base this list on information in RS and other guidebooks, websites and so on. I also make note of restaurants or shops that catch my eye. Once I am actually on the tour, I am thus prepared to fit these possibilities into my free time based on the daily schedule provided by the tour leader (usually posted one day in advance), my mood and energy level, the weather, what other people might be doing of interest and so on.
I only make specific plans if someplace requires a timed ticket or reservation because of demand. This is not necessary for most places. Sometimes it’s just fun to wander a neighborhood and be open to spontaneous experiences. Sometimes I need a rest!
Also keep in mind that Rick’s wonderful guides usually provide an orientation at each location and offer many good suggestions about possible activities, restaurants, cafes, and entertainment. Occasionally they will offer to take anyone interested to a particular favorite place. For instance, our guide in Rome took about half the group to an excellent out-of-the-way Indian restaurant for a change of pace from pasta. In Naples, our guide took those interested to a fun folk-style concert. Another guide arranged for a private concert and happy hour in honor of several tour members who happened to have birthdays on the trip.
As for the opera, if it starts late enough and the farewell dinner starts early enough, you might be able to skip out of the farewell dinner early. You probably won’t be able to find out the exact timing until you meet your guide on the first day of the tour. I usually look forward to this dinner and the chance to have a nice, leisurely meal with the friends I’ve made on the tour, so I would be reluctant to skip out early (or skip it altogether), but you might feel differently.
I almost always add extra days at the beginning and end of tours so I have time to do extra activities or see non-included sights. But I see you are going in September, so it might be too late to arrange this.