Yakima lies in a valley and there is dense fog many of the mornings and it is not unusual for hte morning flights to be cancelled. the six am flight is usually cancelled on foggy days and then the night flights may not arrive because of the fog. the bus is a ten seater so they may not be able to accomodate you. so it is best to fly to seattle and take the bus or rent a car and drive to yakima.
Don't drink and drive. On the freeways abound the town do not speed because they well nail you. In Union Gap don't dare speed a single mile an hour in the school areas. There are certain neighborhoods I'd avoid like most towns. Actually crime is not to bad here. The local people think it is because the papers report everything that happens. That makes it seem higher then it really is. Compared to larger cities it's really nothing.
If you go hiking in the summer, why not go early in the morning, before the sun rises to its full height. There's not much shade out there. And do remember to hydrate!