Planning out a picnic/day for someone whose Kosher

Shabbat Shalom everyone! I know there is likely not a lot of people on here, but anything helps! So I’m planning out a whole day for someone who I really like who happens to be pretty stringent in terms of keeping kosher. Since we are spending basically the whole day together doing active stuff, I know were both going to be pretty hungry. I don’t have a kosher kitchen, so I can’t really cook him anything (unless I am being silly and there are ways to ensure I am still making something kosher for him). Although, say I were to go to the grocery store and confirm that the ingredients I got for a sandwich are Kosher, how would I ensure that in my assembly process and storage of the food that I am keeping it Kosher? Also, I wanted to know if anyone knew of any restaurants/food places in the San Diego area that are kosher. While I could go out and buy snack’s that I know are Kosher, I really want to make sure that we have substance for the day. So sorry if these are silly questions, I’m just personally not Kosher and I want to ensure that he can eat whatever it is I make/get for him!