Expresses that something is pleasing, impressive, or of high quality. Often used in polite or slightly formal contexts.
This is a splendid garden, isn't it?
結構な贈り物をありがとうございます。
Thank you for the lovely gift.
fine; all right; enough; no, thank you
Indicates that something is sufficient, acceptable, or that one does not need more. Sense 3 is the polite refusal usage (e.g., 'I'm fine, thank you').
おかわりはいかがですか？—いえ、もう結構です。
Would you like another serving? — No, thank you; I've had enough.
この部屋の広さで結構です。
This room size is fine (sufficient).
Adverbial use meaning 'to a considerable degree' or 'more than expected'. Often softens a statement.
この映画は結構おもしろかった。
This movie was quite interesting.
彼は結構早く着いた。
He arrived fairly early.
structure; construction; framework
Noun meaning the way something is built or organized. Less common than the adjectival/adverbial uses.
この建物の結構は非常に頑丈だ。
The structure of this building is very sturdy.
I'm fine without more tea, thank you.
It's fine to just leave it as it is.
I received a lovely souvenir.
This problem is quite difficult.
けっこう is a casual, everyday way to say 'quite/fairly', while なかなか carries a slightly more formal or admiring tone, especially in なかなかの〜 constructions.
結構 can mean 'quite/fairly' like 割と, but it also has separate meanings like 'fine' or 'enough' that 割と does not share.