example I:
Then I remembered, almost as an afterthought, to add clove and cinnamon.
(edited) example II.
The vice presidential candidate tends to be a bit of an afterthought.
google translates afterthought as раздумье, but that can't be right.
There's also ретроспектива (retrospect) but it's not a good fit either
"Afterthought" is a word that's highly specific to English, in that it has a strong connotation of lesser importance or relevance, which isn't inherently connected to the idea of not thinking of something until later. Russian is probably far from the only language to not have an established term for that particular combination of ideas.
Russian has its own highly idiomatic verb which, connotation-wise, may be the opposite of "afterthought": спохватиться, "to suddenly [and almost belatedly] remember or realise". The implied importance here is greater, not lesser.
As for your example, I'd rephrase it as Потом добавил(а) гвоздику и корицу — чуть не забыл(а). (Since I think I can safely presume that я was used in the preceding sentence, the pronoun drop here makes it flow better.)
"Afterthought" is indeed has no direct translation to Russian, particularly not in the sense of "thought". However, an action of adding something of a lesser importance to an already compiled list can be described by a number of words and idioms:
I think the closest translation to Russian we can achieve using the transgressive of поразмыслить - поразмыслив.
Затем, поразмыслив, я вспомнил(а), что нужно добавить гвоздику и корицу.
