I envy you.
This is a very good simple free translation! You can replace いいなあ with いいなぁ (small あ) or with 羨ましいなぁ（うらやましいなぁ）.
Here is the grammatical structure.
いいなぁ → 良いなぁ → 良いですね = That is good, indeed.
So, いいなぁ can be used in either of the following cases:
- Just describe the quality.
- Compare someone’s possession with the speaker’s, and then the speaker says “yours is better than mine”. Therefore, いいなぁ means “I envy you”.