Lo que más me gusta de viajar es descubrir un monumento nuevo en cada ciudad.

Lo que más means what ... most or the thing that ... most.

So:

• Lo que más me gusta de viajar = What I like most about travelling

Here, lo que introduces an idea, not a specific noun. It is a very common pattern in Spanish:

• Lo que quiero = What I want
• Lo que necesito = What I need

Adding más makes it what I like the most.

Because gustar works differently from to like in English.

Spanish structures it more like:

• me gusta X = X is pleasing to me

So in this sentence:

• me gusta literally means is pleasing to me
• me = to me

That is why Spanish uses an indirect object pronoun (me, te, le, nos, os, les) with gustar.

Examples:

• Me gusta viajar = I like travelling
• Te gusta España = You like Spain
• Nos gustan los monumentos = We like monuments

Because the thing being liked is treated as a single idea.

In this sentence, the subject is essentially:

• Lo que más me gusta de viajar or, more conceptually,
• descubrir un monumento nuevo en cada ciudad

That whole idea is singular, so Spanish uses gusta.

Compare:

• Me gusta viajar = I like travelling
• Me gustan los monumentos = I like monuments

If the subject is one activity or one idea, use gusta. If the subject is a plural noun, use gustan.

In Spanish, the infinitive often works like a noun in English.

So:

• viajar = travelling / to travel

In de viajar, the infinitive names the activity itself. Spanish does this very naturally:

• Fumar es malo = Smoking is bad
• Leer me relaja = Reading relaxes me
• Lo mejor de viajar... = The best thing about travelling...

So viajar is not a verb being conjugated here; it is being used as the name of the activity.

Because de here means about or of, depending on how you translate it.

• Lo que más me gusta de viajar = What I like most about travelling

This structure is very common:

• Lo mejor de vivir en Madrid... = The best thing about living in Madrid...
• Lo difícil de aprender español... = The difficult thing about learning Spanish...

So de viajar means about travelling or of travelling, depending on what sounds most natural in English.

Because it is being used as the complement after es to explain what that favourite thing is.

The sentence structure is:

• Lo que más me gusta de viajar es + infinitive phrase

So:

• ...es descubrir un monumento nuevo en cada ciudad
• ...is discovering a new monument in each city

Spanish often uses the infinitive after es when defining or identifying an activity:

• Mi hobby es leer = My hobby is reading
• Lo mejor es descansar = The best thing is to rest / resting
• Su sueño es viajar = His/Her dream is to travel

In Spanish, adjectives usually go after the noun, so monumento nuevo is the neutral, standard order.

• un monumento nuevo = a new monument

If you say un nuevo monumento, it can sometimes sound a bit more literary or can suggest another / an additional monument depending on context.

So in this sentence, un monumento nuevo is the most straightforward choice.

A useful general rule:

• adjective after noun = the normal descriptive position
• adjective before noun = often adds emphasis, style, or a slightly different nuance

Here nuevo most naturally means new in the sense of different/newly encountered from the traveller’s point of view.

So the idea is:

• in each city, the speaker enjoys discovering a monument that is new to them

It does not necessarily mean the monument was recently built.

Spanish often lets context explain this. In a travelling sentence like this, listeners will usually understand nuevo as new for the speaker.

Because en means in, and the phrase refers to location.

• en cada ciudad = in each city

The speaker is saying that when they travel, in every city they visit, they discover a new monument.

Cada means each and is followed by a singular noun:

• cada ciudad = each city
• cada día = each day
• cada vez = each time

Because cada ciudad focuses on the cities one by one: each city.

• en cada ciudad = in each city

If you said en todas las ciudades, that would mean in all the cities, which sounds more total and definite.

So:

• cada = each, one by one
• todas las = all the, as a complete group

In this sentence, cada ciudad is the more natural choice.

Some parts can change, but the original order is the most natural.

For example, these are possible:

• Lo que más me gusta de viajar es descubrir un monumento nuevo en cada ciudad.
• Descubrir un monumento nuevo en cada ciudad es lo que más me gusta de viajar.

Both are grammatical, but the first version sounds more natural as a neutral statement.

What you should keep together is:

• lo que más me gusta
• de viajar
• descubrir un monumento nuevo
• en cada ciudad

Changing the order too much can make the sentence sound heavy or unnatural.

Yes. That would also be correct, but it means something slightly different.

• Lo que más me gusta de viajar... = What I like most about travelling...
• Lo que más me gusta cuando viajo... = What I like most when I travel...

The first talks about travelling as an activity in general.
The second focuses more on what happens during your trips.

In many contexts, both work, but de viajar sounds a bit more general and elegant here.

Yes, descubrir does mean to discover, but in everyday Spanish it can also mean to come across, to find, or to discover for yourself.

In this sentence, it suggests the pleasure of encountering a monument for the first time, not necessarily making a scientific discovery.

So it has a natural travel meaning, something like:

• discovering a new monument
• finding a monument you did not know before
• coming across a new landmark

That makes descubrir a very natural choice here.