What do you do when you go out in public/day trip?

I breast feed, pump, and occasionally supplement with formula. I am not really comfortable BFing in public yet because I use nipple shields and it makes it difficult to coordinate because LO bats them off a lot of the time.

I prefer him to have breast milk over formula, but I am always paranoid to take bottles of pumped milk in case he doesn't eat them and then they will go to waste.

What do you usually do? BF in public? Pumped milk bottles? Or use formula when you are out and about?