No doubt your Yaris will serve you well, as these cars are well made and very reliable, thus cheap to keep running.
Pump up tires, do a grillblock, but not more that cooling is OK for your urban driving. Also go to low rolling restistance tires when it time to change.
The Yaris will respond well to at bellypan.
Move antenna inside.
Take out back seat and clear car for stuff that is not needed everyday, as for instance spare tire (Depends on the roads good/bad). Lower weight will help MPG in urban driving.
Seal up, or tape areound hood and headlight / bumper/fenders for better areo.
Get a set of smoot hubcaps (Pizza pans) and make covers for rear wheel wells.
Good luck. Gotta love them Yarises...
