Suggestions for relaxed activities in Rome with teen

I went to the top of the Victor Emmanuel monument about half an hour before it closed, had it to myself and lovely views over Rome. I entered via the Capitoline Museum precinct, through a lovely old church and then took the elevator. As I recall Rick includes the directions to get to the top in his Rome guide. On a sunny day the light in late afternoon is the best.

Can recommend the ruins in the basement of San Clemente church, Roman streets and temples. And the church is lovely. There are a number of ruins close by in a park like setting and nice views of the Colosseum.
Ostia is a wonderful choice as it doesn’t get overly busy. Not sure about the catacombs for him, they tend to be busy and I don’t find them particularly interesting but then I am usually fighting claustrophobia!

Unfortunately, the best items to see in Rome (The Sistine Chapel and Coliseum/Forum) need to be pre-booked.
Try doing them in the morning.

Other things are
The Pantheon
Trajan's Markets
Capitoline Hill Museum

Note - as a typical teen, my son is NOT a morning person, so early morning experiences are a no-go. Sunset, twilight, or after-dark experiences though - absolutely!

Also, a personal connection with a guide would be great. A couple of years ago we took a free walking tour of Athens, and my son still talks about the guide and how much he enjoyed that experience. It was the interaction and the engaging storytelling that were so memorable, rather than the sights themselves, I think.

a personal connection with a guide would be great.

Ruth, problem is that most guides also need to be pre-booked, and their fees are usually non-refundable if you cancel day-of. If flexibility is a must, then you need to rely on attractions that don't involve any advance entry fees or pre-reservations, or those that have easily obtainable tickets on the spot.

A suggestion for an underground tour that doesn't need to be pre-booked? Take a look at the catacombs of San Callixtus or San Sebastiano out on the via Appia Antica. You can walk in (on the days/hours they're open) purchase a ticket, and they'll put you on the next English-speaking tour. Wait times vary. These are very, very old and very interesting subterranean burial grounds. A prior thread from earlier this year with my links:

https://community.ricksteves.com/travel-forum/italy/catacombs-in-rome-3fe5a1cd-718f-4829-98c7-36f4f41344a8

While you're out on the Appia, you might want to explore a bit more of it? Most of what's along its length is free or have just small entry fees, Another recent thread with links:
https://community.ricksteves.com/travel-forum/italy/via-appia-antica-appian-way

I'll echo the vote for Ostia Antica: easily done on whim and weather! Rick has a self-guided audio tour and map (has one for the Appia too).
https://www.ricksteves.com/watch-read-listen/audio/audio-tours/italy

Yes, for the high-up view go to the Victor Emmanuel monument. No advance tickets needed.

Rent a surrey or bike to explore Villa Borghese? Fun for all ages and range of abilities, and there are a couple of rental places in the park (Bici Pincio; Ascol.)

Churches, churches, churches! The biggest bargain in Rome 'cause almost all are free, and almost all of the them are interesting in one way or another. For instance, I'll send you to very old lady Santa Sabina, up on the Aventine (free), for a look at a church that still retains its layout as a 5th-century basilica. The Orange Garden next door affords a nice view of Rome, and there's an interesting view of the backside of the Palatine on the walk up the hill. Peer through the keyhole in the gate of the Priory of Malta while you're up there too,

https://www.turismoroma.it/en/places/basilica-saint-sabina-allaventino
https://www.turismoroma.it/en/places/savello-park-or-orange-garden
https://turismoroma.it/en/places/villa-magistrale-sovereign-order-malta-aventine

Another underground adventure would be San Clemente: a church built upon the foundations of an earlier one, which in turn was built upon a Mithraic temple. The issue is that pre-bookings appear to be mandatory now but in March it may be possible to do that some morning in Rome - whilst waiting for your son to get up - if there are still openings for that day.

https://www.basilicasanclemente.com/eng/

The Colosseum does have two outdoor ticket booths for visitors who didn't make reservations in advance but I would make that slugabed get up early to stand in line with you; tell him if he REALLY wants to get inside without prior reservation, he'll have to rise and shine! You both have to be physically together to buy tickets, have legal IDs on you, and be able take whatever entry time they give you. I would guess that they do not sell tickets for the underground; those are the most in-demand ticket in Rome, and they sell out online within minutes of availability. Whatever you do, do NOT buy tickets from random individuals on the street. All tickets are nominative, meaning they'll have your ID info on them to prevent scalping.

Your young person will be free but they'd provide an entry ticket for him.

https://colosseo.it/sito/wp-content/uploads/2023/11/Biglietteria_Ticket-Office_Colosseum-scaled.jpg
(This is the official website but is only active the hours the colosseum and associated attractions (Palatine, Forum) are open)

Just a start? :O)

One more....
A young man of his age would probably find the 'bone church' of the Capuchin Crypt of interest. I'm not personally a fan as the catacombs are much, much older + much more interesting, IMHO, but here's the website (use an online translation tool). It does involve tickets but those do not have to be purchased in advance unless booking a guided tour... which is not really needed for this one.

https://www.museoecriptacappuccini.it

Actually, the ticket to enter the Forum/Palatine Hill - Forum Pass Super - is not a timed entry - it's good for any day/time within a 30 day window:

Admission to the Roman Forum-Palatine archaeological area and Imperial Fora
Entry to SUPER sites
Entry to any exhibitions in progress at the Roman Forum-Palatine archaeological area
Open date tickets valid for 30 days from purchase date with the exception of every first Sunday of the >month and other free admission days

So you would have the opportunity to go when the time was right. However, you would need to purchase in advance.

Also, the view down to the Forum from the viewpoints accessible from the piazza by the Capitoline Musem is another way to preview the Forum, after walking up the Michelangelo Steps.

Also check out the Stadium of Domitian, which is located below Piazza Navona. You could think of it as a mini Colosseum - an introduction to Roman stadiums with a smaller time commitment.

Largo Argentina would be another option - you can preview from above and then decide whether to venture down and pay the small admission fee. Plus the cat sanctuary is there.

Rome is great for walking around at night, with the ruins fully illuminated. Take a stroll down Via dei Fora Imperiali to the Colosseum.

markcw, the timed-entry ticket I referenced is for the Colosseum, not the Forum + Palatine.