How to answer Let's

Question

Let's go see a movie

If I can go, I can say, “Alright, OK, sounds good.”
If I can't go, I say, “I can't go, I have to...”

Right?

Answer 1

Let's is a contraction of Let us so, in your example sentence, the person is really saying "Let us go see a movie." It is indeed a suggestion to go and view a movie together.

Note that, although it is phrased as a command, cultural convention is to interpret it as a suggestion.

So, to answer your question, either response would be fine.

Answer 2

It's a simple suggestion to which you can answer with:

• Let's
• Alright or Right
• Okay or OK
• Sounds good
• Sure thing or Sure

If you don't want to go:

• Can't, sorry
• Sorry, I cannot go
• Unfortunately I can't go