Hi, Giangilberto from Milan, Foro Romano, Colle Palatino, and Circo Massimo are all within walking distance.
Vicus Caprarius, the water city beneath the Trevi Fountain, is a fifteen-twenty minute walk in the opposite direction.
The only area that's far away is Trastevere, on the other side of the Tiber. That deserves a half-day of its own, preferably in the afternoon or evening.
The Foro Romano, Colle Palatino, and Circo Massimo together are already a full half-day, and the Colle Palatino, especially now with this infernal heat we're experiencing, could be tiring because it involves a lot of walking.
Just a little tip: if you decide to walk to Trastevere, remember that the cobblestones of Monti and Trastevere are the most tiring part of the day for two people your age, more than the distances.
I'll leave you three useful official links below. The first has all the information you need to visit "Vicus Caprarius." The second is the page dedicated to the Colosseo, Foro Romano e Colle Palatino and the third one is for the Circo Massimo:
https://www.vicuscaprarius.com/en/opening-hours/
https://colosseo.it/en/tickets/colosseum-roman-forum-palatine/
All the following links are for the Circo Massimo, including the vivaticket website where you can buy the tickets
https://www.turismoroma.it/en/events/circo-maximo-experience-2026-immersive-tour-circus-maximus-augmented-and-virtual-reality
https://www-sovraintendenzaroma-it.translate.goog/content/circo-maximo-experience-2026?_x_tr_sl=auto&_x_tr_tl=en&_x_tr_hl=en
https://museiincomuneroma.vivaticket.it/en/tour/circo-massimo-area-archeologica/2473