One thing you can do, is
cheat!
Instead of playing one
chord, you can allude to it or play a suspension. One trick that works quite well is to play the 'home note' (eg, C) and the fifth (G); then you have the freedom to play around with modal interchange and dramatically change the feel whilst still playing 'in key'. Joe Satriani calls this
Pitch Axis, there are a bunch of other names as well.
The 3rd is what gives the chord its power (arguably; at least it's a big "I'm and X chord" sign), and is also the hardest to veer off of once it's in the chord. Playing just the root and 5th allows you to go from a major third to a minor third without changing the underlying chord. It also allows you to shift the root note and make it a different scale degree entirely as long as you remember to play the same 5th note that's in the chord (unless you're intentionally going for the rub).
Edit: This is is a variation on what
eclecticsynergy stated above; also good advice, though it will sound more like a key change than just a subtle shifting of emotion...