The Colosseum Underground tour is amazing. It is probably too late to book it through the Colosseum in English as they sell out fast but you can book it through a tour company. We went through real rome tours and could not fault the guide at all. It was well worth it. There is also the Vatican where you can get a guide tour through the Vatican direct, audio guide or guided tour by a tour company. If you don't go through a guided tour then the line can long to get in. Check their opening times especially for Sunday's. There is also the catacombs if you enjoy that kind of thing.
Venice you can easily walk around and see what there is to do. There is St Marks, you can go to the other islands and see glass blowing etc.
Of course there is also the food too.
Trip Advisor has some good suggestions as well but it all depends on your interests.