T.C.
Sun screen (spray on and face rub on)
Lip Balm
Bug spray
Swim diapers
Rash Guard/Swim suit
Towel
A change of clothes to something lighter
A frozen teething ring (in the diaper bag) - I use this for my 3 year old when he gets too hot. He loves it.
I do the same thing, I have a tub in my trunk that I keep swim gear in and a change of clothes. I actually got this together this past weekend.