Learn Farsi: گل‌آرایی یک هنر جالب است. - Flower arrangement is an interesting art.

It is a compound noun made of گل (gol, flower) and آرایی (ârâyi). The second part is built from the present stem of the verb آراستن (ârâstan, to arrange or adorn) plus the noun-forming suffix ی- (-i). Literally, it means the act of flower arranging.

Why isn't there an ezâfe on یک to connect it to the noun?

In Farsi, numbers and the indefinite article یک (yek) come directly before the noun they modify and never take an ezâfe. The ezâfe is only used afterward to link the noun هنر (honar) to its adjective جالب (jâleb), giving us یک هنر جالب (yek honar-e jâleb).

Since we are studying numerical approximation, how would that work if I wanted to say 'one or two interesting arts'?

You would simply place the next consecutive number right after یک without any conjunctions, creating یک دو هنر جالب (yek do honar-e jâleb). Remember that because Farsi uses a singular noun after all numbers, هنر remains singular even though the approximation implies more than one.

Could we express 'an interesting art' without using the word یک?

Yes, instead of the standalone article یک, you can attach the indefinite suffix ی- (-i) to the end of the noun phrase. This would give you هنر جالبی است (honar-e jâlebi ast). Both forms are perfectly natural and are often used interchangeably.