Here are mine, so far:
Not sure if the US and UK models have these, but here in Canada, you get these cheesy bright yellow and offensive warning/instructional stickers, written in two languages, on the visors. Horrible, right in your face. First week I had the car, I had to cover up those! I used a product I use in graphic design and signage, AveryŽ White Permanent Durable ID Labels for Laser Printers 6579
They are highly durable, have a high tack, resistant to elements (even used them outdoors in rain and direct sun).
Just make your own design using your design software. I would recommend making them 2mm or 1/8" wider and longer to fully cover the existing label and it creates a bit of a beveled look. Very tacky... place slow and sure, you only get one chance!
Time: 45 min from design to print, trim, to application.
Rear light added. This was a must for me, with tinted windows, black interior, there was never enough light at night. Found a push button LED for $6.00. I painted it flat black and stuck to the head liner with one side of industrial Velcro (only a few cents).
OK, a puffy smiley face sticker... not a mod, but defiantly cheep and everyone who rides with me smiles at it/comments on.
Price: Free (took off a birthday card)