I live just outside Zillah. Cherry Wood is a neat place, nice people. That will be a good time to be there as it won't be near so hot. Lot's of wineries close.
Fresh Hop Ale Festival in Yakima is October 1st. Lot's of great beer. Google it for more info.
Hop Museum in Toppenish (just accross the river from Zillah) is good for a few minutes while you are close.