Interlaken, Switzerland to Naples

Flying to Naples from Zurich or Basel is probably the easiest way.

This is really too far for a train all the way.
I would fly from Zürich (or Geneva or Basel) airports (train from Interlaken to all three).

• Swiss flies Zürich to Naples.
• Easyjet flies Zürich, Geneva or Basel to Naples

I've just noticed. Your question says:

We will be in Interlaken and are going next to Naples on 17 July.

But the title of the post says:

Interlaken, Switzerland to Milan

Which do you mean, Milan or Naples? They are at opposite ends of Italy.
The answer for Milan would be different, get a train from Interlaken.

Thanks Chris F, sorry I have just re read this and it is confusing.
We are going from Interlaken all the way to Naples, I was just thinking that getting to Milan airport would be easiest then fly to Naples. Any recommendations would be appreciated :)
We are in Australia and I am trying to look online and it looks quite hard to get from Interlaken to Milan with lots of change of trains then from Milan central station to an airport??
We are novice travellers, so if you know how easy/hard this would be we would love to hear.
thanks so very much.

No way train Interlaken to Milan then fly. From Interlaken, fly from Zürich, Basel or Geneva, as I say above. All can be reached by train from Interlaken.
Interlaken Ost - Zürich Flughafen (airport) = 2h14
Interlaken Ost - Basel airport = 2h22
Interlaken Ost - Genève-Aéroport = 3 hours

Easyjet is probably the best option, as it is a cheap airline. Check flights and prices from all 3 airports: https://www.easyjet.com

If you were going to Milan, a train is quicker than flying:
Interlaken Ost - Milano Centrale (city centre) = 3½ - 4½ hours
But as you say you would then need to get to the airport , but that is not much longer, as you don't have to go into Milan, you can take a short cut.

"lots of change of trains . . . . novice travellers" - changing trains seams to intimidate many novice travellers. It is not a problem.

And if you happen to miss your train connection then just have a seat and wait for the next train.

To find train options, use the Bahn (German rail) website, as described by Rick in this tutorial: https://www.ricksteves.com/travel-tips/transportation/trains/online-schedules

To find flight options, use Skyscanner, then book directly with the airline: https://www.skyscanner.com/. Be aware that many of the flights within Europe will have strict baggage restrictions and extra charges - read all the fine print carefully and make sure you understand it, so there will be no surprises at the airport.

Since you are dealing with mountains (trains can only go around these, unless a tunnel has been blasted through) and two different countries, you have to make several changes when taking the train from Interlaken to Naples. These changes are easy; it's not like at an airport where you go through security. You just get off the first train and on the next one. Sometimes it's right across the platform, but often you have to go down to a passageway and then up to your next platform. In turn, sometimes there are ramps, elevators, or escalators to take you from the platform level to passageway level, but often you have to take stairs.

For more on trains, the rail guru The Man In Seat 61 has it all, but his information can be overwhelming at first. Start here with his introduction: https://www.seat61.com/Europe-train-travel.htm

In your situation, we would take the trains. You can do it in 8 to 9 hours with 2 or 3 changes, not bad for such a long trip. I think flying combined with train to the airport, security wait, inevitable delays, and transport at the arrival end, it will take all day anyway. I looked at www.thetrainline.com for schedules. It is not cheap... but you are traveling a long distance!

For the naysayers, I’d prefer to spend the time ont he train with more room, the ability to move about at will, choose my meals (even bring a picnic on boars), and so on. We arrive relaxed, whereas when flying we arrive stressed and annoyed. Sometimes train days are a nice respite from sightseeing. Read a book, watch a video, nap, catch up on your journal, even talk to your partner!