First time in Italy - best tour booking sites?

I should clarify that I want walking tours or day tours.
But, even booking a tour of the Colosseum looks so challenging with so many options.

Get Your Guide is a reseller. Nothing wrong with that, necessarily, but a lot of people prefer to book through the tour company's own website.

In Italy, we used The Roman Guy (aka The Tour Guy). We were really pleased with them. In fact, we had booked them for 2020, and when Covid hit, they offered extra credit for those who wanted to wait and reschedule instead of getting a refund. We chose to do this, and we got two extra half-day tours with our extra credit.

We used The Roman Guy for a tour of the Vatican (including the Sistine Chapel and St. Peter's), a trip to Pompeii and Sorrento, a tour of the Capuchin Crypts, the Catacombs, and a bit of the Appian Way, and, in Venice, a tour of St. Mark's, a few other spots in Vienna, and a gondola ride. We were happy with all our tours with them.

https://theromanguy.com

We booked the Colosseum through the official "Coop Culture" site. It WAS challenging, because it involved getting up very early a certain number of days beforehand to book the tour we wanted, which included the underground. But once we had that booked, the rest was easy. The booking site/process has changed since then, so I'm not sure if my experience still applies.

If you share a list of venues you are interested in we can provide official links

hey hey CanTraveler
when is this trip? how many days/nights in each city? know that check-in times to hotels is 2-4pm and check-out is 10-11am, untile you get early check-in. if not drop bags at hotel, roam around at go back at check-in time.
if you are planning this year, know that it is jubilee2025 (holy pilgrimage) in rome. expecting millions of people from all over the world. will be crowded to be expected and many may travel to florence which is always busy. be prepared
there are many sites to check out, depending on what you want to see and do in each city. many look at the site itself to book, i've used getyourguide.com (a reseller) in 4 or 5 european cities and never had a problem.
cenacolo.it
the last supper in milan, book early. many places have been booked up for months. many tickets are date/time stamped, skip the line does not mean skip security. everyone goes through that line
museoalfaromeo.com
since husband likes cars, this is about 10 miles from milan. take the train and bus, spend couple hours and back to milan
discoverwalks.com
has some cities you're visiting, some free some not
gpsmycity.com
put in city and they offer self guided tours.
ask about other things/tours you're hoping to do, do some research, come back here and ask. these posters here are just volunteers willing to give info (good bad & ugly) for you to have a great time and know ahead of time with your plans. depending on your dates in italy may be too early to be booking some tours
keep asking questions and will help you out. happy travels
aloha

I always look at get your guide even if I found the tour elsewhere. I like how the site breaks down information, and I find the prices are occasionally cheaper there than on the company's official website. I would use get your guide for research purposes and then double check reviews of the company and look at the company website before booking.