There are plenty of traditional ways to decorate a wedding car. However, if you know the couple getting hitched has a good sense of humor and appreciate unusual things, it may be a bit more challenging to decorate their wedding car appropriately. These are a little out of the ordinary, just like the couple getting married.
Using window markers or other water-based media, write funny sayings on the car’s windows, including the couple’s pet names for each other and inside jokes. You can refer to the new husband having a brand new “ball and chain” and use perennially apt phrases for describing the wedding night (but do keep it clean). This kind of decorating can get you into trouble with the wrong sort of couple, so use only when you’re sure it’s all right.