A customised and revitalised four year old Trek District
The CDX system in a 55/20 configuration - 10% quicker than the original gearing on the flat
As referenced in an earlier post, you should also consider replacing the original tyres with something much tougher, such as Continental GatorSkins. For predominantly urban riding, you may also want to replace the wide handlebars with a short, straight bar, while swapping out the leather grips, which are prone to unravelling, for something hardier. And it also makes sense to add toe straps to the pedals to keep your feet in the right riding position.