Translation:Let's get ready for some fun.
'Lets" is incorrect. lt needs to be "let's" which is an abreviation for "let us". Therefore "let us" is correct
Would this still have the same meaning if you said "Vamos preparar para algum divertimento"?
No,that's because you get "yourself (oneself) ready" for something, so we use the reflexive pronoun...
Not really.. seems fony to say it like that. The closer I can think is Vamos nos preparar para se divertir. But we would use something more specific, like para sair (go out), para ir no cinema (go to the movies). Nos preparar would also be more specific, like nos vestir/arrumar (get dressed).
We don't. This sentence sounds very unnatural to me, as a native speaker.