Visited both Rome and Venice a few weeks back so here's a few suggestions I hope helps;-
In Rome, as mentioned booked tours, private/small tours are best, and you will save ALOT of time bypassing all the waiting queues. Vatican Museum, Sistine Chapel and St Peter's Basillica are must do's. (TBH I felt underwhelmed with the Colleseum but do visit and tick the box). Then other sites around Rome which I highly recommend are:- Saint I'gnazio Basilica, Vittoriano/Monument. Then there's all the other tourist sites one would visit like the Trevi Fountain, Pantheon, the many Piazza's. If the weather's not too warm you could basically do a whole walk around the main sites of Rome in one day. FWIW we stayed at the Westin Excelsior.
In Venice, hmmmmmmmmmm, firstly expect boatloads of tourists LOL That was one of the biggest turn off for me. However the biggest negative were the Italians .... rude, arrogant, inconsiderate, and I can categorically say that are the worse of the worse when it comes to all my travels around the world. [I swear I could have ended up in a fist-fight with one of the waiters in a restaurant just around the corner from the Rialto Bridge ... but smartly I left quietly and peacefully]. Anyway, canals are fantastic, hidden alleyways, boutique shops are the main attractions here - and that's all there is sadly. We stayed at the Hotel Danieli so we were pretty much smack bang in tourist ground-central ... close Piazza San Marco and every tourist thing around there. If you do want to visit the San Marco Tower though, get there around 30-45minutes before closing time and not only there is very little wait, you also get to see the sunset over Venezia.
Depending where you stay and which port you arrive in from, I also highly recommend a private water taxi transfer. It cost EUR100 for a 25-30 minute ride from VCE airport but flying across the lagoon and onto the Grand Canal in your own private water boat was fun, and then hopefully onto your hotel's private jetty. Remember the movie The Tourist and Angelina Jolie arriving at the Danieli in a boat, that's how it felt.
Buy a 12 or 24 hour water boat pass and cruise up and down the Grand Canal. It is also recommended to cruise up the Grand Canal during the day to see the daily activities, and then at night so you see the amazing lights/chandeliers within the houses lined along the canals. Also catch the boat to Murano Island and see the intricate making of the renowned Murano Glass.
Anyway, have fun, hope you don't get ripped off by an Italian in Rome or Venice.... as much as they are famous for Ferraris ..... they're also famous for ripping us poor tourists off.