Typically when you say birchas hamazon you insert either if your in your own home parents home or in a guests house. What do you say if you are in a park for example or any other public area?

If one is in a park or any other public area he does not have to say ‘birkat haOreach’ (the guest’s blessing) in the birkat hamazon. If one eats in a restaurant he doesn’t need to say the birkat haOreach because he pays for the food (Mishnah Berura (201:7). (However, according to Rav Yaakov Emden, he should say it there because he actually received the food from the restaurant , see piskei teshuvot, 201;4). In a park, in contrast, one didn’t receive the food from any person such as the mayor of the town