Tires, without a question would be the first thing I would change. As a matter of fact, it was the first thing I DID change on my bike as I had the bike shop swap out the tires before I even got it from them. Other things I've done to mine? New stem to change my riding position, better brake levers for more adjustability, better discs (less squeal, better looking), headlight, taillight, road cassette. Admittedly, I started with a more expensive bike than you, but nothing on my bike is really top-of-the line.