We came with the 2-prong adapters, and they worked just fine for us. We charged our iPhone and laptop, and camera batteries with it.
Be sure your electronics are dual voltage! Many are - the newest laptops, smart phones, tablets, etc. My son brought his Game Boy and it was not, so he couldn't charge it.
Don't worry about blow dryers - most hotels provide them (or just buy one when you get there).
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