You’ll need the map to keep you oriented, the compass to keep you moving in the right direction, and the watch to keep track of the sun, so you don’t get stuck setting up camp in the dark.
Prepare for the rain
Nature loves irony. If you forget to prepare for rain, you can be sure that you’ll hit a rogue rainshower at some point. Whereas your clothes may dry as soon as the sun comes out, your map may not be as resilient. If you only remember one thing, have it be a waterproof map.
The quickest way to spoil a good backpacking adventure is being stuck in the mud, soaked from head to toe, with the sun setting and the map quickly crumbling and dissolving between your fingers.