Try a drop D tuning. All this means is tuning the bottom (6th) string down a whole step (the equivalent of two frets, or half-steps) to the note D, instead of the usual E. The interval consisting of the low D and the A on the next open string up (the 5th string) is known as a fifth, and is part of any major or minor triad (the penultimate basic chord). You also have the octave of D on the open 4th string. With this tuning, the guitar has a fuller sound compared with how it sounds in standard tuning. The drop D naturally frees you up a bit when you are playing in the key of D (or in other keys up the neck, using a capo). There are a lot of songs written in “drop D” tuning.