As a wedding photographer I see tons of cars decorated.
First thing I can tell you is not to do anything that will get them dirty/messy or really be inconvenient. (Not that I think you would ... but just as a reminder.) At one wedding, the groomsmen shrink wrapped the entire car with packing plastic (like the kind movers use to wrap around furniture). The bride and groom had to spend 45 minutes cutting it off their car at 1 in the morning before they could go on to their hotel. Not a good idea.
I would keep it simple and fun.
- White shoe polish in one of those bottles with the foam applicator (do NOT get this on the finish or the body). Use it to draw hearts on the windshield and write "just married" on the back window.
- White streamers - esp if they have a luggage rack, tie long streamers of white crepe paper from the luggage rack so they trail out behind.
- Balloons - not many, but tie a couple to the antenna
- You can also stuff the body of the car with balloons (pref. white ones) - this is less annoying than most things, cause balloons can be popped pretty quickly with the car keys
That's about the most I'd do ... if it were me. Enough to be fun, not so much that it's annoying.
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