epltd has said it all insofar as traveling companions are concerned.
Some of us, generally travel freaks, go many places where others fear to tread. Ultimately, we are contacted by people we love and who want to share time with us in foreign locations, enjoying the travel experience. It is a compliment to us, and since we love these people anyway, why not blend our vacations and show these neophytes the ropes?
Not necessarily a bad idea, but expectations can be different. When you are the travel "planners," you deny your friends one of the best experiences of a vacation--the planning and anticipatory pleasures which can make a holiday special. So, in the interest of giving voice to expectations, and with a "been there--done that" sensibility, I suggest that you gift your friends with a copy of RS's Italy guide and maybe a Lonely Planet guide as well. Let them do their own research into destinations and sights which interest them. I am still overwhelmed on all my vacations by the myriad of adventures to be undertaken, and when your friends come to you saying that they REALLY want to see Michelangelo's statue of Moses, or the armory in the Doge's palace, pay attention and assist them in finding other places and things.
People who are bosom friends back home are sometimes unbearable "on the road." Their insecurities manifest themselves in surly comments about how expensive this all is, or how "if I have to see another painting of the Annunciation I'm going to scream." I try to rise early and get a jump on the day's activities. If that is you, then get a commitment from the friends that they will do the same thing. Plan "go your own way" days, perhaps meeting up over dinner or an after dinner gelato. Make sure that they understand that because this is YOUR vacation, as well, you don't want to be responsible for all the nuts and bolts.
Have planning meetings increasing in frequency and details as you approach your departure date. Discuss things like packing, cash vs credit cards, jeans vs slacks, dressy vs comfortable. Try to achieve consensus when you can; learn that you will not dovetail completely with them in this strange new environment.
Having said that, Ravenna is phenomenal. So are the Michelangelo sculptures in the Medici Chapel. So are sgroppino or an Aperol spritz at a canal side table in Venice. So is taking off your shoes in a park after a hot day walking through the Eternal City. Have fun, enjoy your friends, but make them do their fair share of carrying water.