If you have steel wheels on the MEsa Runner (it looks like they might be chromed steel in the catalogue page but it is unclear) then switching to alloy rims will instantly make the brakes work better - especially in wet conditions where steel rims can be downright scary. Also aluminum wheels are generally more durable than steel wheels, not to mention lighter. That and new brake pads like Koolstop Salmon pads will make the bike safer. CHanging cables and housing (if the ones included on the bike are more than a couple of years old) will cost $15 or so and make a quick improvement to the brakes and ease of shifting.
If you are riding mostly on road or hard surfaces, then narrower high pressure tires will make riding easier and probably more enjoyable. A decent floor pump ($25 - $30), if you don't already have one, is a very good investment - underinflated tires are a common cause of flat tires, and flat tires are a common cause of getting frustrated and quitting riding.
Last edited by DCB0; 04-19-12 at 07:43 AM.