just buy a pair of 6x9s but make sure they are shallow enough to fit, trim the front doors and stick em in. a 3 way decent pair of 6x9s should give you more than enough volume, clarity and a little added bass, but in about as simple of an addition as you can get (short of just getting a good pair of 6x8s or 5x7s, which as people already said, would work fine)
when shopping for this size sometimes its tricky to find though, thats another reason i suggested 6x9