Every year people descend on the sleepy eastern Washington town Prosser, WA to watch The Great Prosser Balloon Rally.
Hot air balloon rallys are a great chance to get some beautiful and unique photographs. What is especially great about the Prosser rally is that due to the typical wind conditions, the balloons almost always cross right over the Yakima River which you can see makes for some wonderful reflections. The balloons take off from the Prosser Airport and usually land just east of town. The most popular spot to watch from is the main bridge over the Yakima. The bridge will be full of people on both sides. This is a great spot to photograph from if you can find some room with the crowds. I'd definitely NOT plan on using a tripod with all the people there.
My favorite place to shoot from is the end of 9th street by the Prosser Falls Education Center
To shoot here you will need a flexible zoom lens say from about 50mm to 200mm. There will be other people there too but in my experience not too many. You may want to photograph from the bridge on one day and from the end of 9th street on another day too. You may also want to photograph at the airport earlier in the morning to get closeup photos of the balloons being prepped like this one.
There is also a night glow that is fun to attend where you can capture photos of the balloons lit up. Just be aware that unlike other rallys I've been too, you can't get right night to the balloons and there are only five or six balloons. This is just due to the space constraints as it's held in a high school football stadium.
Of course eastern Washington has big open skies as well an you can have some fun with night photography if you'd like as well.
Just remember that the hot air balloons only fly if the weather is just right. I was there one day when the weather looked great to me and everyone expected the balloons to fly but it turns out there was moderate wind shear. If you go, I'd plan on being there the entire rally to maximize your chances of seeing the balloons fly.