Guitars are pretty good percussion instruments. Drumming and tapping on the various areas of the body and neck will yeild drum tones of various sorts. Scratching muted strings has its place too.
When playing chords which require you to mute certain strings, for example the open D chord, the muted low E string will lend a definite thud with each strum if it's not avoided altogether. This isn't bad, it just colours the sound differently. Same if the low E string is muted while playing an open C chord. Sometimes by concentrating on strum patterns, and the strum effort, on the lower pitched strings, this will give a very percussive sound to your playing.
Use the above technique alternating with fretted lower bass notes for those D and C chords, (F#, and G or E respectively), and you've really spiced up your strums.