It has far too much wonder and so many hidden gems and something amazing around every corner that it should be banned!
In July it will be absolutely roasting, so brace yourselves for that. Early mornings and late evening are best obviously. You will melt during the day so be careful. I was there late September into the first days of October and it was still very very hot. No need to buy bottled water at every shop you see. Just bring your own container/bottle and refill at the Nasoni which are public drinking water fountains dotted everywhere, and the water is great from them.
I am late 60s and walked everywhere on my own. I just had a look at all the sites and plotted a walking route that would take me from the metro station I got off at to a metro station at the other end to connect back to my line back. It worked out great. Very tiring in the heat, but so worth it!
Over three days I did the ancient parts around the Colloseum. I didn't go in, the queues were too long, but I was happy enough to see this iconic building from the outside and the area around it such as the Forum, the baths of Caracalla, and anything else that looked interesting around there.
Next day started off at Piazza Navona and walked from there all the way across via Pantheon, Victor Emmanuel, Trevi, and the absolutely amazing church of St. Ignatius Loyola with the amazing ceiling that you look at via a mirror in the nave. Wow.
Third day was to the Vatican for the Papal audience. OMG, the crowds were just unreal, but it was well organised and the atmosphere was electric when IL Papa came out on the balcony. For me it was something not to be missed. That took most of the morning, so I was exhausted.
Last thing I would recommend if you have time and energy - I took the train from Porta San Paolo station out to Ostia Antica to see the ancient Roman city. OMG it is as good as if not better than the likes of Pompeii. Honestly. And no crowds, water fountains, toilets etc. It was amazing. I self guided with info from the ticket office. Highly recommend.
I was totally exhausted at the end of every day, but it was an absolutely amazing experience. I felt very safe, was careful on the metro and in crowds, and used a spring coil to attach my phone to inside my bag, and clips to attach the outer zip to the shoulder straps. No one was getting MY stuff ha ha. But it can be very hairy, since you are totally enraptured by what you see around you and can take your eye off the ball, then they swoop!
So sorry for the long essay, but I enjoyed it so much I can't help my enthusiasm!