Both of these depend on what the question before is. If the question before this isn't along the lines of "Would you like some...?" then this translation might not work. In Japanese, if you want to agree with the question, then you would use はい for agreement and いいえ to disagree. However, if the question is a negative one, you would use はい to agree with the negative idea and いいえ to disagree with the negative.
For example: (positive question) 今日の天気はいいですか? Yes- いいです. No- よくないです. (negative question) 今日の天気はよくないですか? Yes- よくないです. No- いいです.
The thing is, if you mean to say any of the following: "That would be nice" "That sounds good/great!" "I would love to" all indicating that you are responding in the affirmative, you don't say "いいです。" If you say that, you will just get the opposite reactions than you expected. This is because the Japanese interpret the phrase "いいです。" as a way of turning down an offer. In English, this phrase might be the equivalent of "No, thanks, I'm good" or "No, thanks, it's fine." Notice that in Japanese, you don't reply in the negative when turning down an offer as that comes across as too direct and negative, and therefore, rude.
I learnt this the hard way.
So when asked at the checkout counter if you need a plastic shopping bag, "レジぶくろはいりますか。" and you forgot to bring your reusable eco-bag, you reply with "はい、お願いします" ("Yes, please") if you DO want a bag, or "いいです" if you don't. The latter means "No, it's fine (I don't need it)", and not "Yes, that would be fine. (I'll accept it.)" Don't be like me, who many a times ended up awkwardly gathering my purchases in my arms, while the cashier and I gave each other confused looks as I walked away.
いい is a very confusing word. It can mean "I want something," "I don't want something," or "it is nice." If you explicitly stated the subject +がいいです then it can be nice. It is tricky.
- お天気がいいです。 The weather is nice.
- 空気がいいです。 The air is nice.
Normally to make it less confusing we add ね if we want to say "it is nice."
- そのケーキの味がいいですね That cake tastes nice.
- そのアイディアがいいですね。 That idea is nice.
Now if it is in the form は いいです。 then it is inclined to mean "I don't want." If it is が いいです then it sounds that you want it.
- お茶はいいです I don't want tea.
- お茶がいいです I want tea.
- 袋はいいです I don't want a bag.
- 袋がいいです I want a bag.
- それはいいです I don't want it.
- それがいいです I want it.
If you just say いいです then it is inclined to mean "I don't want it." If it is いいですね then it would be "It is nice."
Hope you can understand this tricky bit. If there are any word that can be used instead of いい, use that word to avoid possible confusion, like 味がいいです=おいしいです.