You might need everything.
Average high is 71. Average low is 51.
Its been known to be in the 30's and its been known to be in the 80's.
This past February when I went it was about high of 82 each day.
I wore shorts the first day but was freezing in the evening.
I wore pants the rest and was pretty hot from 11am to 4pm.
Don't decide on any one thing until a few days out.
Even then bring some variety.
Some people advise wearing a bunch of layers to shed but personally I don't because that's all the more you have with you in the parks.
My suggestion is pick the bottoms you will wear for the day, wear a t-shirt, and have a zip up light jacket.
I have no problem just aking off the jacket and tieing it around my waist for a few hours.