Help me with 4 day Rome itinerary

[Roma12345]

As title says, please help me with 4 day itinerary.
2 adults, 3 teens. ( 15 - 19)
Lazio area...airbnb
Thinking:
Day 1 : Early flight, check in to airbnb and a wander around
Day 2: vatician city
Day 3: colesseum / roman forum etc but this prob only takes 3 hrs ?
Day 4: ??
Day 5 eve flight home
Help me plan it better please.

Are there any activities for teens / cruise on Tiber/ vespa tour etc...that aren't too expensive ??
Any recommendations welcomed please.
By the way.....Aug so I know it will be hot, very hot but for all 5 of us to all to get away together dates these days are they only ones that work

One of my tips for anyone going to Rome in summer - get up and out early, go back to have a siesta after lunch (especially if you’ve got air conditioning) or plan for air conditioned museums in the early afternoon (12-4 especially) then head back out late afternoon for more sightseeing before a late dinner. The middle of the day can be brutally, brutally hot as you’re aware. Count on lots of rest and make sure to drink water (lots of water fountains about).

Make sure you book the Vatican in advance or you may have to queue - and it can be in the middle of the square in direct sunlight.

Ideas of things to do:

Appian Way and the catacombs
Castel Sant Angelo (nice and cool in the middle because it’s underground)
Pantheon is a must
Basilica of San Clemente (also underground)
Domus aurea (also underground)
Outdoor parks - Villa Doria Pamphilli, Villa Borghese, Park of the Aquaducts
Ostia Antica & trip to Ostia (on the coast)

Vatican can be teamed up with Castell Sant Angelo, Vatican museum and then go wander around Trastevere.
Coliseum/forum/plus Bocca Della Verita and Victor Emmanuel monument
Trevi fountain/pantheon/Spanish steps (must see even though can't sit there) and general wandering.

You could spend all day in Colosseum/Forum , across the road to Palatine museum and there is a Roman house below street level, plus Circus Maximus. Above the Colosseum is a park where Nero's Golden House is (open selected dates) and remains of baths etc.

Other Roman parts to explore include Baths of Caracalla, Argentine forum, Pantheon, Castel Sant Angelo, . Monte Testaccio,

Art galleries and park at Villa Borghese

Spanish steps and Trevi fountain, Piazza Navona can all be visited in an evening with plenty of restaurants around

Pop into any churches with open doors as you pass by, many have additional chapels or levels with Renaissance art or Roman remains

Bone chapel & catacombs are a must.
We did Vatican and said we would never do it again. It's something you have to experience to appreciate it tho I guess. Airport security style going in, was literally cramped like sardines with tour guides holding up flags so you wouldn't loose them. Had no time to stop to look, as it's a constant flow of people, so you constantly need to be on the move. Armed guards shouting at people collectively in Sistine Chapel to be quiet and constantly reminding you that you aren't allowed to take photos. Could only really relax and take it at your own pace in St. Peter's Basilica. It is very cool seeing where the Pope lives tho & where he does his speech's.
Trevi Fountain is obviously a must, that was great.
We also stumbled across the most random place ever that turned out to be one of the best days : Terrazza del Pincio in the centre of Rome (you'll have to google maps it when you're there) - You'll see lots of people looking over on a balcony, head up to it and what's up there is like another world! Massive gardens/electric bikes, pedlo's on water, restaurants, art gallery, ice cream vans, the list goes on. We had so much fun on the electric bikes, can fit lots of people on them too as they have different sizes. The balcony is amazing to get a photo on, as it's kind of like a viewpoint of all of Rome.
We loved the Colosseum! That was amazing, we regretted not booking a tour guide for it.
Oh do enjoy I'm so jealous!

Day 4 go to Ostia antica.

Lots of the things people have already suggested, plus the Capitoline Museums.