Verona

Well Julliette's "Balcony" can be seen in 5 minutes on a stroll through the old city, so that is a non-issue. March would be a tad chilly for a float trip on the Adige. Lake Garda would be nice, especially the north end. Do you have a car, or will you be using buses and trains?
Boat service is reduced on Lake Garda until the last weekend in March, so boats go only half way up the lake until then.
Venice, on the other hand, would be a great day trip.

bus or train. Wife and I have been to Garda before. I speak Italian, and so was hoping for something a bit unusual for the typical tourist.

The amphitheater to worth a tour, Roman ruins abound...under the street(covered by clear glass), shopping at Piazza erbe and along the road leading from erbe to Piazza Bra is fun. There are several museums in Verona as well as a couple of very interesting churches. I can recall the names right now but I recall one museum being highly rated.

Murray,

It could be a bit chilly around Lake Garda in March. One thing you could look at for the teens is Gardaland amusement park.

https://www.gardaland.it/en/

I didn't check opening times in March but that should be on the website.

We just stayed 5 nights in Verona this past October, I really enjoyed the city on our second stay there. The history is fabulous, the food delicious and the buses/trains very convenient. We went to Venezia for the day (train) and to Lago Garda for the day (bus from Piazza Bra to Garda and Bardolino). We were supposed to go to Mantova for the day, but just enjoyed walking around the beautiful town. There is so much more to Verona than the R/J thing.
In March, as it was noted earlier, it might be a little chilly, but you never know, just be prepared with layers. When our kids were younger and they traveled with us, I had them look up the towns so they would be the "guides". With them involved in the planning and trip itself, they would really enjoy the time and made it fun for all. Good luck!

You can get from Verona to Bolzano in as little as 1-1/2 hours. Bolzano has the museum housing the Iceman as well as a very pretty historic district that looks sort of Austrian.

Teatro Romano and Giardino Giusti (if the weather is nice) might be good options.

DJ

We were in Verona in April 2014, and I think of it as a nice place just "to be" and Juliet's balcony takes just a few seconds to look at - and you can make fun of the tour bus people! "Romeo's" family cemetery is pretty interesting though.

I liked how you can easily walk from the train station into the center of town. The Roman ruins are approachable and not overrun with tourists. Every restaurant we ate at had had horse and donkey on the menu. Your teens might enjoy telling their friends about that!

We made the scheduling mistake of spending our only full day in Verona on a Monday. All of the museums were closed. With that said, we had a great time just wandering around.

We took a day trip to Lake Garda by train. Although it was pretty, to me the lake is better suited for longer stays than day trips.

Have a great trip!

Google Verona Card. It's a museum pass and lists most of the sights in the city. While Juliette's balcony is a nothing, it was kind of interesting to tour the inside of a typical well-to-do house of the middle ages and it's included so no extra charge. The Arena is lovely, especially in late afternoon sun. Wander through the market, then up the tower for the views. The San Zeno Church and the Castelvecchio were not typical. I saw a number of teens climbing up on the bridge. There's something for everyone in the museum there.

Rick's walking tour of Verona is a great way to cover all the main sights. It's in his guidebooks.

We spent a week in Verona this past summer. Our kids loved the Romeo and Juliet play/walking tour which occurs every evening. Ferrari and Lamborghini museums are nearby. We did a day trip to Garda, but not so exciting for them. Climbing the tower and touring the Arena was fun for them.

I'm planning to take a 3-hr bike ride tour when I return to Verona this summer. We've taken bike tours in other towns and find them to be very fun, combining a leisure bike tour with interesting facts and quick stops to see sites. Usually the small group has a combination of age groups.

Here's another vote for the Castelvecchio. It's a great medieval fort/castle with tunnels and turrets and ramparts and more, great for exploring. AND it has an amazing collection of paintings. Great view of the Adige as well.