"Let's meet at the park soon."
Translation:곧 공원에서 만나요.
Could you please expand on why thsts a better choice if you don't mind? Thank you!
Unfortunately verbs ending -자 are casual speaking. The next formality would be 만나요. The next would be 만납시다. So it's hard to tell without context in this case.
만나자 is now accepted as a legit answer, but you cannot say one is preferable to the other without context. This exercise is in 해요체 skill, so the best translation here is 만나요.