Welcome Mike, first, no one can give you an intention, that's for you alone to come up with. I'm sure you can think of something. Do you have anything you would like to achieve or any qualities you would like to improve upon? It doesn't even have to be for yourself, maybe your intentions could go towards helping someone or something else. It could be as simple as giving thanks or gratitude to the trees and plants for providing food and oxygen (as well as that mind expanding molecule).
You are on the right track when you speak of mushroom man guiding and protecting you while you purify yourself in the shower. And focusing on your breath is a great exercise.
For most people intention is best facilitated with ceremony and ritual. It helps put force and commitment behind the intention. The type of ceremony you perform is also completely up to you. It can involve singing, chanting, dancing, drumming, meditation, breath work, art, crafting, fasting, dieting, offerings and maybe most importantly prayer. It can involve pretty much anything. Part of my ceremony is walking in nature on the day I journey. It is great for opening my awareness and igniting my creativity.
The important thing is that your intention ceremony should have meaning for you. It needs to come from the heart, which is the portal to the soul. Follow your intuitive nature, the mind thinks, the heart knows.
There is nothing real scientific about any of this and some may shrug it off as needless or even silly but I like to believe that what you get out of something is in direct proportion to what you put in. I guarantee you will learn something about yourself if that is what you intend. Plant the seed, tend the garden, and it will bear fruit.
Good luck Mike and tell your Pop that the road to heaven is paved with good intentions as well
As Within, So Without.