April with 15 year old son- need itinerary ideas

From a weather standpoint, I'd prefer to head south. Nighttime lows probably in the 40s (F) don't say "vacation" to me. Mid-summer trips to southern Italy can be marred by unpleasant heat, but with an April trip you wouldn't need to worry about that.

We did a spring break trip over Easter in 2019 to Rome, Florence, and Venice with two teens. Their favorite place in Rome was the Colosseum. We would have visited it twice if we had time. They really enjoyed Pompeii and Sorrento, they loved all the food throughout the trip, and couldn’t stop talking about the gondola ride in Venice. They even really enjoyed all the museums. You’ll have a great
time.

If you decide to go to Florence and Venice in addition to Rome, I can highly recommend 2 activities our teenaged grandchildren enjoyed in 2019. In Venice they took a gondola rowing lesson and really got a workout from this company https://rowvenice.org/. In Florence they took a countryside bike tour followed by a cooking class from MaMa Florence. https://www.mamaflorence.com/en

Last point first. Yes, use a multi-destination search function to fly into one city and home from another. It will save time and be no more expensive than a simple round trip when all costs are considered.
More general point: To find out what a teenager might enjoy, ask now. Then work together on the plans -- he is surely old enough to start computer searches for research. Then take some of his suggestions through mutual agreement. Make it his trip, too. He will enjoy learning more about both Italy and life.

I second rowvenice.org. Great experience. Along those lines, when you’re in Florence, you may be able to rent kayaks for paddling on the Arno river.

I wouldn't be too concerned about weather in making the decision. We were in Tuscany one December right before Christmas and never needed more than a light jacket. Lows were upper '30s heating up to the low to mid 60s. April will be even warmer.

I could make an argument for either direction out of Rome. Get your son involved in the planning and let him research and have some ownership in the decision.

BTW, the first time I was in Europe in general and Italy in particular was when I was 15 and on a school trip. There were 8 of us, 4 boys and 4 girls. from our school and 64 students altogether. We were the youngest, most were high school seniors. One of the girls was a cute little freckle faced 14 year old. After a few days we started hanging out together. Then, on August 10, 1966, I got up enough courage to kiss her. We were in Florence. We've been going steady ever since and are coming up on our 51st wedding anniversary.

With only 8 nites, I'd do Rome and Naples , Amalfi area. If you decide on Florence or Venice, fly into either one first. Do keep in mind it's a 4 hour train ride from Venice to Rome.

Florence & Rome combine well, and with this itinerary, you could actually fly RT Rome. Then train immediately to Florence. Ending in Rome for your last days.

If Pompeii is on your radar, RT Rome, then train to Naples and fly out of Rome.

This bike trip along the Appian way might be a good activity.

https://www.ecobikeroma.it/home_eng/

In Naples, this underground tour.

https://www.lanapolisotterranea.it/en/naples-underground/

All of these are great ideas so far. Any more suggestions for travel south from rome

Since Venice and Florence are must sees compared to the south of IT, I recommend flying into Venice and out of Rome and in this order. If you fly out of Venice, you’ll need to get up at 3a to catch your flight home. You won’t have to get up during the middle of the night if departing from Rome.
In Venice get lost and at twilight take the vaporetti (water bus) from the train station to San Marco Square or vice vs. Heck, you can take the vaporetti beyond San Marco if you want. This way you’ll see the exquisite beauty of the palaces that line the Grand Canal and get a glimpse of what it looked like 100s of years ago without seeing the decay on the buildings. Seeing the illuminated foyers and the giant crystal chandeliers is priceless. It literally took me back in time.
You’ll need to buy your Uffizi Gallery (Florence) tickets two months in advance because the Uffizi sells out about then.

You’ll need to buy your Uffizi Gallery (Florence) tickets two months
in advance because the Uffizi sells out about then.

This may not be the case as it may take tourism a bit of time to get up to speed. For instance, I just checked the ticketing site for 2 months from now. - Sept, is normally still high season - and there are plenty of tickets available. Granted, it's early in the country's opening to foreign visitors. But if your son is only into "small" doses of museums, that one might not be a good choice for him, however the terrific cultural experience it may be. That doesn't mean you need to avoid the fabulous art, especially Renaissance art, in Florence altogether - nor should you! - but there are smaller venues that may do the trick without overwhelming him. Take a look at the Bargello, San Marco, the duomo's museum ( Opera del Duomo) and the Accademia, for instance, as well as some of the churches, like Santa Croce (Michelangelo is buried there; a name he surely is familiar with). Do the climb up to Piazzale Michelangelo for a lofty view of Florence. Make a reservation to climb the duomo's dome, or the bell tower. Yep, he might enjoy a bike tour.

We didn't bike the Appia in Rome but we did trek a big section of it on foot: great day! There is a bunch of really interesting things to do/see along the way, although not all of them are open every day.

https://www.parcoappiaantica.it

But your teen might find the Sorrentine/Amalfi coasts + Pompeii to his liking too. There's some fun hiking on the island of Capri, and coastal Path of the Gods (https://www.positano.com/en/e/il-sentiero-degli-dei), plus a trek to the top of Vesuvius (from a bus stop on the upper reaches), and, well, Pompeii is its own adventure....

He's plenty old enough to do some of his OWN research so turn him loose with his laptop and some guide books. What places appeal? What can he find that he'd like to explore, both culturally and actively in those places? What do those things involve? This is a great opportunity for him to start learning how to research trips of his own he'll be planning someday. :O)

After spending decades teaching 14 and 15 year olds I certainly know that no two of them are alike. My suggestions are "in general" and may or may not match with your son.

They relate to history as a story. Any movies that might relate to places you will see will help to set the stage for more interest. Prime the pump before the trip. It helps to know about places/things in advance. If your son is a reader, search out some historical fiction that will catch his interest. Seek out places that have a connection with an interest he has. Into sports? Check out at least one stadium or see a match. Does he like to eat?(This would definitely cover most teen age boys!) Maybe come up with something a little off beat like a cooking class together as a family. Activities? Is there a typical activity that is unique to an area? (para-gliding, walking a pilgrimage path?)

Do a walking tour in Florence - you learn the history and they always have good stories.
I also took my teenage daughter on a Rome walking tour - 2 hours for an overview. Keeps them moving and you get the lay of the land

Walking tours in cities are a great way to learn about the area, history and wonderful stories. I did this with my teenaged daughter in both Rome and Florence. 2 - 3 hour tours - you can ask the guide questions and it’s a good way to get familiar with a city.

Here's another vote for walking tours. Back in 2015, we did the evening stroll of Rome. It was a great way to spend the evening. We also did a morning stroll of Florence, followed by a guided tour of the Uffizi in the afternoon. The tours were by Walks of Italy, which I highly recommend.