How to plan for sightseeing in Rome

Whatever you do get up and get out as early as possible by 8 am.
You can see what the Colosseum and the Forum look like on the inside on you tube and show it to others in your party and vote to see if they want to go in. It does not take hours to get in the Pantheon it is free to enter the line is just a constantly flowing security check point.
Though they may stop the flow to control the crowd inside.
Avoid going to any site between 10am and 3pm.

Note that the Pantheon has an online reservation system for weekend access. Go on Friday or Monday if you do not wish to make a reservation. Also consider the Capitoline Museum which is never crowded.

We just walked into the pantheon about 3 hours ago and no wait. Then walked to Trevi fountain. Walked back to the pantheon and did not notice a line to get in. Definitely prebook the coliseum combo ticket with the forum online. Go to the coliseum first. No wait there because we were a little late. Make sure you know where the entrance is. 5 minute or less wait at the security line at the forum. I wish that we would have booked a tour through the official website. We went to the recreated floor of the coliseum. Very cool. Did not see one person in the underground portion. Maybe they only go in the areas covered by the floor. Hot today. Very few masked people. No cdc shot record check since we got the plane from LHR to Rome. Regular taxi at the taxi stand at the airport to our hotel in the central train station area was 50 euros. Most taxi riders from one site to the other 10 euros or so. Have fun

I'd plan very little and just enjoy Rome. With one weekend, I'd choose one major historical site and try to get early morning or extended evening tour hours.

The Vatican can't exactly be overrated because of what's in there, but less is just so much more sometimes. Kind of like a great quality pizza with friends in a charming spot is far preferable to amazing pasta and wine with rude service, long waits and irritable crowds. I would think you'd actually see more art (and less of people's sweaty backs and heads) and enjoy it far more in the smaller galleries. (I live in Pennsylvania, and I have to tell you, if we had a single work from the "lesser" galleries of Rome, people would travel to see it. In my opinion, there is no less beauty and quality in other sites, just less massive volume. You can't see even half of what's in the Vatican museums in a day anyhow.)

Plus, you can tell people who ask about your trip about things they haven't heard about before!

As you say you'll be in Rome "to" Monday and not "through" Monday, it looks to me like you're only there Friday/Sat/Sunday, and Friday might be a partial, possibly jet-lagged arrival day from the US?

I'll agree with the advice to review some online info/video of the Colosseum/Forum/Palatine and see if they interest you enough to explore more than what you can see from outside. A google will bring up a ton of stuff; here are short lessons on both the Colosseum and Forum in this site, along with some other stuff you might be interested in

https://www.khanacademy.org/humanities/ancient-art-civilizations/roman

Do be aware that even a timed, general-entry ticket for the Colosseum (no guided tour) will include entry to the Forum and Palatine. Booked tours of the Colosseum through coopculture will include them as well but with no guided tour.

https://www.coopculture.it/en/tickets/

There are any number of independent tour companies than offer combo tours which include all 3 sites. They usually last about 3 hours or so; give a should if interested and the folks here can recommend a few.

Yes, currently you need a timed-entry reservation to visit the Pantheon on a weekend but those are free unless booking an audoguide or tour. Here's the website:

https://www.pantheonroma.com/home/

And here is the page for free reservations:
https://pantheon.cultura.gov.it/en/

Note that they must be made at least 1 day in advance or only for the following weekend.

Vatican Museums: you'll need to visit those on Saturday as they're generally closed on Sundays. A tour will help you navigate its miles of stuff (they only cover parts of the museums), and they all include the Sistine. AS with the Colosseum/Forum/Palatine, there are indy companies which provide in-depth tours if what's offered by the Vatican itself is booked up. The other option? Skip them entirely and do the Borghese (Christine's recommendation) instead. Advance, timed-entry tickets for that one are mandatory, they only sell a certain amount for each time slot (which provides great crowd control) and because each time slot visit is limited to 2 hours - they clear the museum after each one - it won't take all day.

https://m.museivaticani.va/content/museivaticani-mobile/en.html

https://www.tosc.it/en/artist/galleria-borghese/

If you haven’t already downloaded it, get the Rick Steves Audio Europe app. There are several audio walking tours that you can take free of charge on this site. They include visits to places like the pantheon in the forum, as well as guided walks through the city. It’s free and it’s very good.

I enjoyed the Colosseum more from the outside, but I don't think you can appreciate the Forum from the outside. In fact, I think it deserves a tour so you know what you're looking at. We used https://www.througheternity.com/ which is the best tour company we've ever used in Europe.