We will be in Rome in two weeks for 3 days. Should we book tours with guides or just go on and get our tickets to the Colosseum, etc? Night or day at the Colosseum? How about the Vatican as well?

I'm not a big fan of tours, but that's me. A tour or two might help you see a greater number of "things" because they keep you moving along... sometimes faster than you like.

I would definitely not arrive in Rome without tickets or skip the line reservations for what I most wanted to see. Much depends on your priorities and how fast you sightsee, but... the Colosseum, the Forum, Vatican, St. Peter's, Galleria Borghese, the Pantheon, perhaps Domus Aurea, Ara Pacis, the Castello...

I'd probably pick the Colosseum at night if only because the temperatures will be lower.

So much to see, so little time.

I agree, if tours are your thing, then you can certainly do that, but if you don't, you want to have tickets booked.

I have a different idea about the Colosseum at night, though. I did it, and it is very dark inside, difficult to see where you're going and difficult to see anything. The light in the arches you see from outside doesn't penetrate inside.

In two weeks? You're going to be lucky to find tickets.

For the Vatican museum it looks like the only slot open on the 26th is 8am. The 27th you've got two choices 8 and 8.30.

Likely the Colosseum will be even harder to get tickets for

Ditto TravelerNick. Your lack of advance planning may limit you to booking access via tours. We used Get Your Guide with great success in Rome for skip the line tours. With only three days, it helps to save hours waiting in line. And keep in mind that if you don't like the tour group you can always take off on your own once inside the site.

I'd be careful with third party ticket resellers.

Partly some sell tickets they don't have . If you're lucky they refund the money. If not you end up getting pushed into buying something even more expensive. More an issue for people buying far in advance but they do it.

Partly the various sites have or are bringing in controls to make sure tickets go to real people and not resellers.