"Let's sing and dance!"
Translation:Laten we zingen en dansen!
Sure that is fine as an alternative translation, but I don't thing it's better. Let's sing and dance! refers to now, not to the future.
I would use it in both situations but I did not think about that it can mean future too so I have to agree that "Laten we zingen en dansen!" is better.
Yup, that's the flexible Dutch tenses, now, future, who cares let's use it in both cases. :)