What is a noun for "nice revenge"?

"requital" is a noun not often used but exact in this context.

• I made them (a) requital.

It is used here for instance: Google Books.

Addition

Here are a few examples showing how the word is combined with the verb "make" or a similar one (from vocabulary.com among a great many instances of use to be found there).

• For all the wrong of slavery requital must be made, submissively, ungrudgingly, repentantly.

• This was worth a requital, and they got it.

• A splendid monument was raised to his memory, on which was inscribed his own epitaph, that no friend ever did him a kindness, and no enemy a wrong, without receiving a full requital.

• Ever since the visit paid to her home by the imperial consort, he fulfilled his official duties with additional zeal, for the purpose of reverently making requital for the grace shown him by the Emperor.

Since we are talking about usage, we are also talking about what people might actually say. Irony is a popular characteristic in British English. So someone might say to a friend who just paid for a meal they shared:

I’ll retaliate next time

or

Next time we eat, I’ll have my own back

Reciprocation is the word you're looking for. In simple words, reciprocation is the act of reciprocating.

Reciprocation: the fact of feeling or behaving towards someone else in the same way as they feel or behave towards you. [CED]

Reciprocation often has positive connotations. Retaliation is often used in the same contexts with negative connotations.

You could also say return the favour.

For example, if someone does you a favour, you can reciprocate by doing the same thing (good for good) or you return the favour.