Drive out to the bonneville sign (the one full of bullet holes) and park your car to walk around barefoot in the salt when it's covered by a few inches of water. Really cool. Bring a hat and plenty of sunscreen if you are spending any time here. Also DO NOT drive on the salt near the area of the sign. Someone was very stuck in the soft muddy salt when we were visiting and it looked miserable. The only way you may be underwhelmed by the Bonneville salt flats is if you have been to the Salar de Uyuni in Bolivia.
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