We just bought our (2006 base) Matrix last weekend, so I'm slowly going through the car ...):
* Replaced the O-ring on the timing chain tensioner ... had a very small seepage of fresh oil.
* got new front tires ... the old BF Goodrich were howling!
* replaced a bunch of underbody clips which were missing ... should quiet down some of the noise in the front end on the freeway.
* replaced the driver side power window switch trim ... old one was chipped on the corners
* replaced the cabin filter ... old one was filthy, probably never changed!
Still to come:
* PS fluid flush, clean out dirty residue from inside reservoir, fresh fill with Valvoline Dexron
* ATF fluid flush through cooling lines ... I expect about 8 quarts will do it.
* find a decent set of wheel covers ... the car has three different types on it now and they're all scuffed up
* install power side mirrors ... currently has manual
* install fog lights ... currently has none, and those black blanks look horrible.
* install Pioneer DEH-6600BT head unit, Polk DB speakers all around, and plenty of dynamat to turn this thing into a Bentley
Things I'd like to do but probably won't for now:
* alloy wheels ... this is a college car for our son ... he'll have to earn the alloys
* have body dings and scuffs fixed ... again a college car for now.
* add leather wheel with cruise control, and leather shifter
Things my wife would like to see done to the car:
* white side mirrors and door handles ... to match our white Sienna XLE
I have to say, after owning the Matrix for only a week, it's a really fun car to drive!