Lo que más recordaré de este viaje no será el vuelo, sino la tarde en que vimos la catedral, el castillo y el mar desde el mirador.

Here lo que means what or more literally the thing that.

So Lo que más recordaré means:

• What I will remember most
• or more literally, The thing that I will remember most

This is a very common Spanish structure:

• Lo que dices = What you say
• Lo que quiero = What I want
• Lo que más me gusta = What I like most

The lo does not mean it here. It is a neuter article used to talk about an abstract thing or idea.

Because más is modifying the idea of what I will remember, not the future ending itself.

Lo que más recordaré is the natural Spanish way to say:

• what I will remember most

Spanish often places más before the verb in this kind of relative clause:

• Lo que más me gusta = What I like most
• Lo que más recuerdo = What I remember most

In English, most often comes after the verb or near the end. In Spanish, it commonly appears earlier in the clause.

The speaker is talking about how they will feel or what they will remember in the future, after the trip is over.

• recordaré = I will remember
• será = it will be

So the sentence is not just describing the trip now; it is making a statement about future memory:

• What I will remember most from this trip will not be the flight...

Spanish uses the simple future here very naturally, just as English does.

This is a very common contrast structure in Spanish.

No ..., sino ... means:

• not ..., but rather ...
• not X, but Y

In this sentence:

• no será el vuelo, sino la tarde...
• it will not be the flight, but rather the afternoon...

Use sino when:

1. the first part is negative, and
2. the second part replaces it with the correct alternative.

Compare:

• No fue Juan, sino Pedro = It wasn’t Juan, but Pedro
• No quiero té, sino café = I don’t want tea, but rather coffee

This is a very common learner question.

Use sino when the second part corrects or replaces the first after a negative statement:

• No será el vuelo, sino la tarde...

Use pero for a more general but:

• Quería ir, pero no pude = I wanted to go, but I couldn’t

So here pero would not be the best choice, because the speaker is not just adding a contrast. They are explicitly saying:

• not the flight
• but rather the afternoon

That is exactly what sino is for.

De este viaje means from this trip or about this trip, depending on context. Here it is best understood as:

• from this trip

So:

• Lo que más recordaré de este viaje = What I will remember most from this trip

Spanish often uses de where English uses from in this kind of expression:

• lo mejor de la película = the best part of the film
• lo que recuerdo de aquel día = what I remember from that day

Este means this, referring to the trip the speaker is talking about now.

Yes, cuando would also be possible in many contexts:

• la tarde en que vimos...
• la tarde cuando vimos...

But en que is very common and often sounds a bit more formal or written.

Here en que means:

• when
• literally something like in which

So:

• la tarde en que vimos la catedral...
• the afternoon when we saw the cathedral...

This structure is very common after words referring to time:

• el día en que nos conocimos = the day when we met
• la noche en que llegaron = the night when they arrived

Because it refers to a completed event in the past.

Vimos is the preterite of ver:

• we saw

The speaker is referring to one specific afternoon during the trip, and the sightseeing happened as a finished event:

• we saw the cathedral, the castle and the sea

That is why the preterite is the natural choice.

If the sentence were describing background, habit, or an ongoing action, another tense might be used, but here it is a completed moment.

Because Spanish often uses definite articles more regularly than English.

So Spanish says:

• la catedral
• el castillo
• el mar

Even though English might sometimes say:

• the cathedral, the castle and the sea

or in some contexts just list nouns more loosely, Spanish normally keeps the article with each noun.

This sounds natural and complete in Spanish, especially in a list of specific things the speakers saw.

Desde means from, in the sense of a point of view or starting point.

So:

• desde el mirador = from the viewpoint / from the lookout point

A mirador is a scenic lookout, viewing platform, or place with a good view. This word is very common in Spain, especially in travel and landscape contexts.

So the idea is that they were standing at the viewpoint and from there they could see:

• the cathedral
• the castle
• and the sea

Because Spanish often omits subject pronouns when the verb ending already makes the subject clear.

• recordaré already means I will remember
• será refers back to lo que más recordaré
• vimos already means we saw

So Spanish does not need:

• yo recordaré
• nosotros vimos

Those pronouns are only added if the speaker wants emphasis, contrast, or clarity.

This omission is one of the big differences from English, where the subject usually has to be stated.