Cambodia and Italy are pretty different experiences but both are great.
Cambodia is great. I went to Siem Reap to see the Angkor Watts in 2018 to fulfill a buck list item. The Angkor Watts are amazing and require at least one full day. You could really geek out and spend a week on them but after a day it all starts looking the same and you have seen the highlights. The Watts themselves are cities and there are lots of them so you need a car to get around to them all, we hired a personal guide and a driver for $35 for the whole day. They even packed snacks, beer and water. Things are crazy cheap also, full epic meals for less than $10 total for the whole table. Our hotel was amazing for somewhere around $40 a day. Thing about Siem Reap is that it is small though. You will run out of things to do after three or four days, and honestly I cant think of many other places in Cambodia worth spending much time in. I would make this part of a great tour of the area, you can easily catch a really cheap regional flight from Siem Reap to Bangkok, Chang Mai, Koh Samui, Phuket and more. Personally, I would add this to a Thai beach vacation.
Italy is a whole other story, I could go on and on and on about there. Just did Florence and Naples and have been to Rome and Venice a few times. They are all amazing and very different. If you have never been to Italy then you have to check it out.
A few of my other top suggestions are Cartagena, Colombia. It is a romantic city and very fun and affordable. Also a lot closer to home. Buenos Aires and Icguazu Falls are an epic trip combo also, but probably not viable if your trying to group it with the Cambodia travel.