Diads & triads

Roots + 3rds are very very useful.

You can play through a tune with just those notes and pretty much hear the basic harmony of the chord progression.

The cool thing about playing only 1 or 2 notes — fingerings and positions don't really matter. When you add that 3rd note (maybe a 5th, 6th, or 7th), things get a bit less flexible.

(make sure you Learn The Fretboard first though :)