DD2's passport and birth certificate, along with my passport, especially important since she was born overseas and I couldn't get a replacement from the State Department without one of the three items.
I'd already fudged it to get her enrolled in two different preschools without the right documentation, but knew that wouldn't fly with kindergarten registration. I started looking a year in advance, finally decided to get replacements, only to discover that my missing passport meant I couldn't prove much of anything. Sat on it for a bit and eventually discovered them on my desk along with a large stack of blind samples. The size, shape, and thickness of the envelope for the passport was the same as for the blind samples.
Phew!
Advice and commentary on living overseas
DD1 15, DD2 12, and DS 9