Last Monday, on the way to the English class in my town, I stopped by a small park because itI was a little bit early for the class.

When I parked my car at a car park, I noticed a pond filled with lots of lotus flowers.

In British English, "car park" is used, so "car park" is fine. But just in case you don't know, in American English, we say "parking lot," and we don't say "car park."

Last Monday, on the way to the English class in my town, I stopped by a small park because itI was a little bit early for the class.

I understood your sentence, but this flows a lot better

Lotus flowers are mystical, aren't they?

It is proper to have a comma there, but informally it's okay not to!

I thought, I'd like to look at them forever.

With the comma it makes the sentence sound like your thought was "I'd like to look at them forever"

Without the comma it sounds like you thought you'd like to look at them forever, but changed your mind

Last Monday, on the way to the English class in my town, I stopped by a small park because it was a little bit early for the class.

What you've written is correct, but I would phrase it differently: "Last Monday, on the way to my English class, I stopped by a small park because I was a little early."

When I parked my car at a car park, I noticed a pond filled with lots of lotus flowers.

Because you've said "filled with", the "lots of" is unnecessary.

The second I saw it, I was so captivated by the flowers andthat I took a lot of pictures of them without thinking.

or "many photos"

Lotus flowers are mystical, aren't they?

I thoughtink I'd like to look at them forever.

You switched to the present tense in the last sentence, which is why I changed the tense. Unless you mean "I took lots of photos because I thought I'd like to look at them forever."