The National Aviary is a great place to visit for anyone who likes birds/animals.
In addition to viewing the regular exhibits, I highly recommend paying the extra $5 for the sky deck experience. You will see large birds of prey flying right over your head, and the friendly and knowledgeable staff will explain what they're doing. And wear sunglasses!
I also recommend feeding the lories. I think they are the only birds in the aviary that you are allowed to touch. They will come right on your arm or shoulder to be fed nectar. If you wait about 5-10 minutes after the initial feeding time, the lorie area probably won't be as crowded, and you can have a more one-on-one experience with the lories.
The penguins are also a must-see. You can pay extra for the penguin experience, but you can look at them really up close and watch a feeding if you just pay the admission price.
Don't forget to check out the sloth. I don't know what it's doing in an otherwise all-bird zoo, but it's a cool addition.