I never leave for even a 1/2 day at the park without:
-Sun sticker sensors
-Sun screen instant cover spray
-snacks (baggies of pretzels, fruit snacks, puffs, etc things I can't always find to purchase in the parks)
-Purell
-Wipes
-Diapers/pull ups (3 on average if it's diapers 1 back up pull up you never know)
-Sippy/bottle
-Empty Gallon ziploc (this has come in handy 1000 times, I put wet/peed/popscile clothes in it)
-Spare t-shirt at least, been caught buying one too many $24 WDW onesie that was never worn again
-Sun hat
-depending on the season I throw in a thin recieving blanket even when they are toddler's it's nice to have them for shade over the stroller, mopping up messes, etc. They fold up to nothing. I fit it all in a Skip Hop Saddle Bag that hangs from the side of the stroller.