kono kouen hodo kouyou ga kireina basyo ha mettani arimasen.

Here ほど marks the standard of comparison.

The pattern is:

A ほど B はない
= There is nothing / there are few things as B as A

So:

この公園ほど紅葉がきれいな場所はめったにありません。

means something like:

There are hardly any places where the autumn leaves are as beautiful as in this park.

So この公園ほど means to the extent of this park / as much as this park.

Yes, この公園 is already a place, but the sentence is making a general statement about places in the world, using this park as the comparison standard.

So the structure is:

• この公園ほど = as much as this park
• 紅葉がきれいな場所 = places where the autumn leaves are beautiful
• はめったにありません = are rare / hardly exist

So the idea is:

Places that have autumn leaves as beautiful as this park’s are rare.

Using 場所 makes the statement broader and more natural in Japanese.

Because 紅葉がきれい means the autumn leaves are beautiful.

Here 紅葉 is the thing being described by きれい. In Japanese, the thing that has the property often takes が.

So:

• 紅葉がきれい = the autumn leaves are beautiful
• 紅葉はきれい would sound more like as for the autumn leaves, they are beautiful, which changes the nuance and feels less natural inside this kind of modifier.

Since 紅葉がきれいな is modifying 場所, it works like a relative clause:

紅葉がきれいな場所
= a place where the autumn leaves are beautiful

Because きれい is a na-adjective.

When a na-adjective directly modifies a noun, it takes な:

• きれいな公園 = a beautiful park
• 静かな町 = a quiet town

So here:

• 紅葉がきれいな場所 = a place where the autumn leaves are beautiful

Even though きれい ends in い, it is not an i-adjective.

This is a noun modified by a clause.

Japanese often puts descriptive clauses before nouns:

• 私が昨日行った店 = the shop that I went to yesterday
• 景色がいいホテル = a hotel with a good view
• 紅葉がきれいな場所 = a place where the autumn leaves are beautiful

So 紅葉がきれいな is a whole clause describing 場所.

Literally, it is something like:

[autumn leaves are beautiful] place

Natural English:
a place where the autumn leaves are beautiful

めったに means rarely, seldom, or hardly ever, and it is normally used with a negative verb.

So:

• めったに行きません = I rarely go
• めったに見ません = I rarely see it
• めったにありません = there rarely are any / it is rare

In this sentence, it means:

There are hardly any such places.

So the full nuance is stronger than just there are not any. It suggests such places are very rare.

That is just how めったに normally works in Japanese. It is a negative-polarity expression, so it usually appears with ない / ません / ありません.

Examples:

• めったに食べない = rarely eat
• めったに来ません = rarely come
• めったにありません = rarely exist / there are hardly any

So めったにあります would generally sound wrong.

This is a very common Japanese comparison pattern.

AほどBはない literally means:

There is nothing as B as A.

So even though the overall message praises the park, the grammar uses a negative form:

• この公園ほど紅葉がきれいな場所はない
  = There is no place with autumn leaves as beautiful as this park’s
• ...めったにありません
  = There are hardly any such places

Japanese often expresses the highest degree this way.

It is mainly a matter of politeness.

• ありません = polite
• ない = plain casual

So:

• この公園ほど紅葉がきれいな場所はめったにありません。
  polite
• この公園ほど紅葉がきれいな場所はめったにない。
  casual

The meaning is the same.

は marks 場所 as the topic or the thing being evaluated in the sentence.

The core idea is:

紅葉がきれいな場所 = places with beautiful autumn leaves
はめったにありません = are rare

So 場所は sets up such places as the topic:

As for places with autumn leaves as beautiful as this park’s, there are hardly any.

This は is very natural in comparison statements like AほどBなNはない.

That is close in meaning, but it is a bit stronger and more direct than the Japanese.

A more faithful translation would be:

• There are hardly any places where the autumn leaves are as beautiful as in this park.
• Few places have autumn leaves as beautiful as this park’s.

So the Japanese suggests this park is exceptional, but it does not literally say the absolute best in a strict factual sense. It is a natural comparative way of praising it.

Because この公園ほど is not a possessive phrase. It is a comparison phrase.

• この公園の紅葉 = this park’s autumn leaves
• この公園ほど = as much as this park / to the extent of this park

So ほど attaches directly to the noun phrase being used as the standard of comparison.

In a sentence like this, it is usually read こうよう, meaning autumn leaves / autumn foliage in a general sense.

• 紅葉（こうよう） = autumn foliage, autumn colors
• もみじ often refers more specifically to maple leaves or can be used as a common word for momiji

In travel, weather, and seasonal description, こうよう is very common.

Yes. A simpler version could be:

この公園は紅葉がとてもきれいです。
= This park’s autumn leaves are very beautiful.

But the original sentence is stronger and more expressive because it says this park is rarely matched by others:

この公園ほど紅葉がきれいな場所はめったにありません。

That gives a sense of:

You almost never find a place this beautiful in autumn.

A useful way to break it down is:

この公園ほど
= as much as this park

紅葉がきれいな場所
= a place where the autumn leaves are beautiful

は
= topic marker

めったにありません
= rarely exist / there are hardly any

So the full structure is:

[As beautiful in autumn leaves as this park] places are rare.

Natural English:

There are hardly any places with autumn leaves as beautiful as this park’s.