"Let's meet at the park soon."
Translation:곧 공원에서 만나요.
에 and 에서 respectively do not mean in and at. 에 is used when you are going to or existing at/in the place. 에서 is used when you are doing an action in/at the place, like meeting, eating, writing, studying, etc. Living, lying down, standing, sitting, etc. belong to the 에 case whereas sleeping, standing up, sitting down, coming from, etc. belong to the 에서 case.