My grandpa told me not to tell my grandma that we were there.
Does anyone else know you were there? Did you not even tell your mom?
That bitch goes there all the time to annoy him with stupid things. And mom is no less.
It's fine to surprise people from time to time, but this feels more like your grandfather is asking you to lie on his behalf so that he can hide from your other family members which is wrong. Not only should you not keep secrets from your mother and grandmother, but you shouldn't be lying on behalf of anyone else.
I shouldn't have to lie to get time to myself. Also i should be able to do what I want on my own time.
I agree that you should be able to spend your time in your way. Next time let your grandfather know how uncomfortable lying for him makes you feel. He might be willing to understand your point of view.