If you're planning your first visit in Rome, you know how hard is to select what to visit and when. Too many museums, monuments to see..too many "must-sees"...
Based on my experience, (so far I provided more than 400 tours) the best way to enjoy this overwhelming city is to focus your energy for few sites and plan only one monument per day.
Only once in Rome you'll understand how tiring is to walk on the old cobblestones or how far is the Vatican from the rest of the city.
Consider your visit as an introduction: you can tour in the morning and spend the rest of the day, relaxing, having a glass of wine in a nice Roman vineria (winebar).
So for ex, I'd suggest to plan a day for the Colosseum and another day for the Vatican (a fullday would be too intense).
If this your second time in town,or if you're really interested in Art, I warmly suggest to choose a different tour!
Basic tours are warmly suggested even for families with kids (kids have a limited resistance to historical tours)
Roman Forum and Colosseum
Vatican Museums, Sistine Chapel and Basilica of St.Peter
walking tour with the best fountains and plazas of the center.
Discover highlights of Rome with a qualified guide (Ancient Rome + Plazas)