Hello Reza. I live in Rome and have visited venice more than once.
You have a "problem", a little like a child who enters a toy store 😊
I understand you fly into Rome, and will fly out from Venice 6 days later, leaving you five full days to visit Italy.
Here is my suggestion:
When in Rome, take an open, double-decker bus (the London bus style), they are generally hop-on, hop-off, and the ticket will be valid all day. They will drive you along the best sights in Rome and if you're lucky (good weather) taking pictures from the upper floor of an open top bus is priceless!
In the evening, wander among the alleys between Piazza di Spagna, Fontana di Trevi and the Pantheon and enjoy the easy night life as well as an italian lcecream.
I suggest you take the train to Florence and spend one day there.
I strongly recommend the "FrecciaRossa" or "FrecciaArgento", these are superfast trains that will take you to Florence in under two hours.
Similarly, same kind of train to Venice.
There are cheaper trains but they are slowerm, with more stops inbetween and perhaps a change-over in Bologna on your way to Venice.
But, above all, AVOID McDONALDS! You didn't come to Italy to eat a burger, did you?
Tips for eating out:
1) Cappuccino is only in the morning, afternoon cappuccino is the typical telltale sign to pickpockets that you are a tourist - hence a target.
2) Bars are for drinking, you DO NOT sit at a bat table and order spaghetti; you will get a microwave blob of precooked, frozen stuff for twice the price of a proper meal in a trattoria. Similarly, sandwiches may be more expensive than you think, if you're hungry do as the locals, a square cut of pizza on the fly (pizza al taglio)
3) Never order "a glass of wine", they will open a bottle, pour you a glass and make you pay for the full bottle anyway; either you order a caraffe (1/4, 1/2 liter draft) or a "half bottle" (375ccs, or 3/4 pint)
4) If you think some foods look funny or icky, hey, this is where the Roman Empire originated, and we've been eating this "funny stuff" for 3000 years, so ity's got to be good! 😱