A lot of folks recommend a good set of tires as an inexpensive upgrade. My bent came with Kenda Kwests. I replaced them with Michelin Wild Run'r lites and I thought I could tell a difference.

Then I tore them up and installed Schwalbe Marathon Plus, which let me ride over broken glass with impunity.

Edit- just saw the stock tires are Kenda Kwests. Nothing wrong with them, but you might get some better tires at biketiresdirect.