icing on the cake

(the) icing on the cake AND (the) frosting on the cake (something good that is added to another good thing, some attractive feature, an extra enhancement; any unexpected extra or bonus || something that makes a bad situation even worse) — приятное дополнение, приятная неожиданность; ~ бонус || украшательство, что-л. поверхностное, вводящее в заблуждение || ~ это уж чересчур, слишком (ирон.)

Example 1: He was delighted to have his story published – getting paid for it was just icing on the cake. (thefreedictionary.com)

Example 2: I’m really happy playing here, and if we win today that will be the icing on the cake. (MM) —... а если мы выиграем сегодня, это вообще будет просто замечательно.

Example 3: Oh, wow! A tank full of gas in my new car. That's icing on the cake! (thefreedictionary.com)

Example 4: All this theology is icing on the cake... "Pas serieuse"... is an epithet I would apply to the whole book. (‘Listener’, Suppl) — Вся эта теология крайне поверхностна... Pas sérieuse {фр. несерьёзно}. Вот мое мнение об этой книге. (К.)

Example 5: I could handle him drinking too much, but cheating on me? That was the icing on the cake. I threw him out and I don't regret it. (english-test.net)

NOTE The idiom can be used in either sense. It basically means something that added an extreme element, whether good or bad, to a situation. It can either be the cherry on top of the ice cream sundae, or the straw that broke the camel's back, so to speak. (usingenglish.com)

the cherry on the cake (the final thing that makes something perfect)

Example : The fabulous weather on the day was the cherry on the cake. (MM)