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The other thing I'd recommend if it is hot out is bringing 3-4 flat cloth diapers to the park packed in ziplocs, not to use as actual diapers, but as shade and cooling/cleaning cloths. You can drape them over the stroller for shade, dampen them and drape them over the baby as a cooling layer, or worst case, use them to clean up an awkward diaper blowout (you don't want to know how I learned that one the hard way -- suffice it to say that you can also tie two flat diapers together to make an emergency onesie if you run out of clean clothes.)