Welcome!
LBC is right with his "free mod" idea...as soon as I got the car home it was the first thing I did!Some other simple things that you can do without crucifying your wallet are painting your 'stache, brake calipers, and rims. Since you've got 18s and you seem to like the style of them, I'd suggest hitting them with some Duplicolor High-Performance Wheel Paint, or getting them professionally powdercoated...it's something that's really simple but really changes the look of your ride!
LEDs are (relatively) cheap to do, but be careful as you can really go overboard with that stuff! I did a writeup on how to do the basics here:
http://forums.altimas.org/zerothread?id=372632
How do we pay for this stuff? Well, as has been mentioned before, it seems as though alot of owners here still live with their folks and don't have any financial obligations..... *sigh* I'm self employed so when I really want to throw something new on the car I just take on a few extra contracts, work my a$$ off for a couple of weeks solid, and then enjoy the spoils! It also helps that one of my biggest contracts is with one of the top adult websites in Canada........
...gotta curb the spending though...I'm getting married next year and my better half deserves the best for our honeymoon!