Use ‘IN’ when talking about enclosed places.
So, if it is a playground (with swings and slides) enclosed by a fence:
- We spent all morning IN the playground.
The same goes for field: if it is an enclosed one, you can say IN the field, However,
ON the playground and ON the field are both correct
when talking about the SURFACE:
- I found this watch on the playground.
or
when the places are OPEN (not enclosed/defined):
- The kids were running on the playground.
‘AT’ means something like ‘very near’ or ‘next to’:
- Let’s meet AT the playground.
(You don’t have to go inside, but have to be there, very near, so that I can see you if I look at the playground from a distance)
Regarding the BEACH,
because it’s an open area, we never say in the beach. It’s not enclosed, we are on its surface, so the correct form is: ON the beach.
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