I guess there are a few words which spring to mind when I think of the word niceness. friendliness pleasantness, amicability to name a few. I am British and wouldn't normally use that word although it is perfectly acceptable. I think if I was to use a similar word it might be charm or charming.
I think it depends on the usage that you are aiming. If, for instance, you are implying the niceness of the weather then perhaps the word pleasantness would be suitable. However, if you are pertaining to the niceness of the person they you may perhaps use the words politeness or kindness as substitutes.
2-3 synonyms for the word "niceness" along with a few sentences using them:
Kindness - This refers to the quality of being friendly, generous, and considerate towards others. For example, "I was touched by her kindness when she offered to help me with my work."
Pleasantness - This describes the quality of being enjoyable, pleasing, or agreeable. For instance, "The trip was made even more enjoyable by the pleasantness of the weather and scenery."
Friendliness - This denotes the quality of being warm, welcoming, and approachable. For example, "The friendliness of the staff at the hotel made me feel comfortable and at ease."
If, for instance, you are implying the niceness of the weather then perhaps the word pleasantness would be suitable. However, if you are pertaining to the niceness of the person they you may perhaps use the words politeness or kindness as substitutes