Catacombs

I’ve been to several in Rome only. I don’t remember the names, sorry.
If you want to wow your grandkids, and they are old enough, you might want to try this: https://archaeology-travel.com/italy/capuchin-crypt-rome/
I think I did it with Walks of Italy. They used a small van to take us from the museum to the catacombs.

We went to the Catacombs in Paris with our son at the end of his junior year in high school. We all enjoyed it and it was on his list of things to do. We went on the afternoon we arrived but the line was too long to make the last entrance time. We arrived a little after noon on another day and got in line. There was a brasserie across the square and my son and I got baguettes while my wife waited in line. We ate our lunch as the line moved along. The Denfert-Rochereau RER/Metro station is across the square from the entrance. There are no metro stops that close to the Catacombs exit but there are bus stops nearby. We have not been to catacombs in Rome or Naples.

We only did the catacombs in Rome - at St Callixtus. We took the bus out to Appia Antica (bonus - Appia Antica!!) We looked around some ruins but showed up just before they were closing for lunch. So we just walked up the road a bit to see some interesting sights - then came back for around 2pm and got right in - no line, no crowds (I hear Paris can be quite a wait to get in). This was in '08, late Sept, so maybe it's diff at busier times of the year.

Would they wow kids? Hmm - well, there aren't any bones to see...

And we did the Capucin crypt in Rome - that was super creepy and fascinating at the same time!

"Best" is one of those subjective terms. :O)

We skipped the Paris Catacombs as they're just not all that old (late 18th century), don't involve any traditional burial practices, and pre-purchased, timed-entry tickets (to avoid long queue) are very expensive. Likewise we skipped the Capuchin Crypt in Rome as it lacks the age and history of other 'true' Roman catacombs.

Roman catacombs date as far back as early 2nd century. While you won't see any bones in the open niches - practically all of those have been removed from tour routes out of respect - you'll get a look at/background about the hows and whys of subterranean burials, both Christian and non-Christian, depending on the site, during the eras such entombments were active.

Only a few of the known catacombs in Rome are open to the public, all of them require a guided tour (available right at the sites in English), and photography/filming is not allowed, as they're considered sacred cemeteries. We did San Sebastiano, out on the Appia Antica, and San Callixtus/Callisto (mentioned above) is nearby. As above, we just walked in and got our names on the list of the next English tour; the wait was not long. Your graduating "kids" are essentially adults who should be able to engage with the tour content, whether they are "wowed" or not.

I'll agree with the poster above Appia Antica itself is a bonus! While most visitors do not hike as long a piece of it as we did, there are interesting things to see on shorter stretches from the 2 catacombs above; pick a nice day!

Some history on Roman catacombs:
https://en.wikipedia.org/wiki/Catacombs_of_Rome

Websites for S. Sebastiano and Callixtus with hours, tour fees - which are MUCH more reasonable than timed-entry tickets for the Paris catacombs - etc. Note that S. Sebastiano is closed on Sundays; Callixtus on Weds. You do not need any sort of prior reservation.

http://www.catacombe.org
https://www.catacombe.roma.it/en/index.php

Appia Antica:
https://www.parcoappiaantica.it

The Naples catacombs are on our list for a future trip! :O)

There are actually some bones in Rome, I think. We are taking this tour...I’m excited to see the aqueducts, too.

https://www.walksofitaly.com/rome-tours/rome-catacombs-tour/

I think there's a small amount of remains visible in Domitilla but we didn't see any in S.Sebastian and I've not seen any mentions of such at Callixtus. Let's just say that they've all but eliminated the morbid factor, for lack of a better term, in favor of the history, the customs of the time, examples of very early Christian symbolism, etc.

Forgot to mention, you are expected to dress respectfully, as you would for an Italian church, with knees and shoulders covered (this is mentioned in the WOI tour material) or risk being denied entry. It's very chilly in the tunnels so don't worry about overheating! I will also mention that some of the tour content can be a little heavy on Christian myth - for instance, there's no proof that the relics of SS Peter and Paul were ever temporarily kept at San Sebastiano, as our guide claimed - but no matter as the tour was otherwise interesting enough to have been well worth the effort.

Funny they aren't specific about which catacombs your tour will be doing, Lulu? Might be S. Priscilla.