I got new wheel covers like this for mine. There are a ton of options ranging from $25-45.
An OBDII scanner is kinda useful and you can use it with your phone to display info that the Yaris doesn't. Things like MPG, tachometer, etc.
I don't know if this will be useful but I'll throw it out there: wifi hotspot for your car for $20/month. More info here.
Cleaning your car also spruces it up too. Enjoy your Yaris!