General extended chords (add9, 7th chords, 9th, 11th, 13th chords), possibly with altered jazz extensions like #11ths, etc.
Suspended chords, as you said - sus2 and sus4 - possibly voiced as quartal harmony (ie. stacked fourths or fifths). And other voicings in general - doubling unusual notes, inversions or just wide voicings are other ways of adding interest.
Other than that, it's as much to do with the context of the chord as it is to do with the chord iteself. Aside from using normal functional harmony, try also using borrowed or altered chords, from a related scale (ie. playing iv instead of IV in a major scale), and not forgetting about the diminished triad!
Also, using secondary dominants, classical chords like the neapolitan or augmented sixth, and using the tritone substitution instead of the dominant.
There are more jazz or classical substitutions, extensions and similar things, I think, that I either haven't remembered or don't know, but I think what I've mentioned is a good start
I've never heard of adding the fourth to a major triad, btw