This was a tour that we took in the morning, which isn't as hot as the afternoon, and I advise that. We were able to go to the famous sights by foot, with a focus on the famous squares and fountains, and at the end of tour, we ended up at the Vatican. Because of that, I also advise you to take the tour on a Sunday so you will be able to hear the Pope's sermon. Also, if you are wearing shorts, they won't allow you in to the church, so keep that in mind, or you can buy paper pants for about $5.
The walk starts at Piazza Navonna, which is a really big square with cafes, restaurants, musicians, and of course, a fountain there too. We revisited this square on our own at night, so it was vibrant! We walked to the famous Trevi Fountain. Very nice, lots of photos and lots of people there; whether it is day or night, expect lots of people here! We then also went to the famous Spanish steps and went back there at night. It was a hot enough day that it was tempting to swim in the fountain! But always remember: no matter how hot it is, there's plenty of Italian ice cream in Italy!
On a sidenote, we went to Campo Fiori on our own and had the best ice cream ever--the place was called Blue Ice, and it was awesome. The famous Vatican and its piazza attracted a big crowd too, especially since it was Sunday, which was good.
Overall, for a good tour, be comfy and drink a lot of fluids.