'in' or 'at'

My first instinct is "in." Sam is inside the boundaries of the park.

But then as I think about it, I'm not sure that it really matters. You'll hear people say "I'm at the park." Just as often as you'll hear people say "I'm at the mall." or "I'm at the grocery store."

Marius Hancu

IMO, it depends on POV (Point Of View): 

 

I am at the park: I see Sam riding his bike in the park. 

I am at home thinking about Sam, who is out at the park:  Yes,  Sam is  at the park riding his bike, he's riding his bike in the park. 

Position in the city/town: at

Physical motion of the bike, the place where it moves: in 

Hi Marius

I agree with your interpretation of 'in the park' and 'at the park'.

When you're with the children in the park, you would say, "The children are playing in the park."  It would not be correct to say that the children are playing at the park because 'at the park' also means 'outside the park'.

 However, when you're not at home and you know the children have gone to the park, you would say that they are at the park because, IMO, you wouldn't know whether they are in or outside the park.