Erasers - The 3 pictured here are basic to a good art set. If you carry just one, it should be a white eraser (middle right) so you can erase without leaving color behind (like a pink or green eraser). The gum eraser (top left) works on durable surfaces and can erase darker or stubborn pencils marks. It can also pick up masking in watercolor. A kneaded eraser picks up charcoal and can be used as a blending tool. Stick with good art brands and you won't be sorry.
Drawing Tools - You can easily get by with a few good drawing tools: B, and pencils