using in versus on

[lusero]

I am very confused as to when I should use 'in' versus 'on'
for example..I have something in my mind or on my mind.
I like to walk in the park or on the park?

What are the rules for using these two words?

Thank you!

[Tdol]

If you have something on your mind, there's something worrying or making you think. What's in your mind is just waht you're thin king about at the moment or your current opinion about something.
I'd use 'in the park'. With open spaces like parks, gardens and fields, spaces that have a limit, use 'in'.