A couple ideas that wouldn't be too expensive..
LED License plate bulbs: A couple bucks on eBay, just get some white T10 LED bulbs from china and look up the guide on the boards. Takes a few minutes to change them and makes a big difference.
LED dome lights: If you check out KITT222's interior light tutorial, here:
viewtopic.php?f=28&t=37574" onclick="window.open(this.href);return false; it accents the rest of the lighting nicely.
Shift Knob: Our shifter thread is M12x1.25. If you look up M12x1.25 shift knob on eBay, there are quite a few decent options that won't break the bank. I have a polished metal one from Sickspeed, it's got the shifter pattern engraved into it and has a good feel to it when shifting. You can get universal shift boots online too for under 20$.
Carbon fiber vinyl to the interior trim also spruces up the interior a little. You can get rolls for next to nothing on eBay, and once you get used to working with it the possibilities are limitless. Look at DVibe's garage for inspiration, he's done a lot. I had a sample piece so I did up the chrome trim around the climate control knobs. Guides are available everywhere online for this DIY mod. I'm waiting on some more to come in to do the shifter surround.
Super Tone Horns mounted behind the grill also look pretty cool. It's a popular mod with the Subaru crowd but a few of us have put them on our Vibes as well.
As others have said, Debadge
Black out roof rack are also great ideas. Some people have made a "Home-Depot Front Lip" out of some garage door liner, with pretty good results for minimal cost. Again, guides are available on the site. Let's not forget painted break calipers, like Thebarber said.
One more: Window tint.
One other thing you could do that isn't exactly a mod, is to just keep the car clean and shiny. Detail it, show it some love, clean the engine bay, and you'll see the results of your hard work. A set of aftermarket wheels really sets a car off, too, especially with a drop to give it a nice stance.