Sometimes the best things to upgrade are the "little" things that make a big difference. For example, if your bike has a crappy saddle, pedals, or grips, replacing those will make the bike a lot more enjoyable to ride. Tires are another big one. If you put narrower slicks on you could probably lose some weight and make the bike faster and quieter on the roads.
A rigid fork could be great upgrade if you don't go offroading. Besides those things I wouldn't stick too much money into the bike, but rather save up for a better one.