On the tailgate to my 240 Wagon, I have:
Above the VOLVO logo:
A HARDLY ART sticker (Seattle based indie label) and Nipton, California tourist sticker my sister gave me.
Above the 240 emblem:
A little Italian flag and TACOCAT (seattle punk band) sticker.
Next to licence plate:
a NYCPBA (NYPD Police union) supporter sticker that my Grandpa gave me (one you can only get if you are related to an officer).
On the passenger side rear window:
an IPD Volvo/Swedish Flag decal, a vinyl logo for my preferred European car specialty shop.
Thats it, I feel no need to add more. Once these stickers become faded however, I will replace them with similar stickers of similar size. We'll see how they hold up.
related: since my 240 is a 1991 model, I wanted to add a NYPD Operation CAT (combat auto theft) decal to one of my back windows. Operation CAT was a program the NYPD launched circa 1990 that gave officers the right to stop any car with the decal, without reason, between Midnight and 5AM, the idea being it was a theft deterrent for owners who dont use their cars at those hours. When I found this fact out I decided against it, because I keep odd hours during the weekends and driving a 1991 Volvo makes me stand out enough

I had wanted to add one of these stickers because they were on literally every car in New York during my childhood and I thought it would be a nice period appropriate touch.