in 'n out is basically a customize your burger place. its less a secret menu and more "this is what we named stuff people order a lot"
for example you can go in and ask for your burger to have no veggies, no top bun, and only cheese and meat, and they'll do it.
or you can ask for a lettuce wrapped burger, no buns, and they'll do it.
the only complaint i have is its unclear you can order stuff to go on top of your fries, and their fries suck cause they aren't double-fried, but whatever.