My favorite part of this zoo is the big cat exhibit. If they are out, they photograph beautifully in most light, although exceptionally well on slightly overcast days like this one. I only had my 105 mm lens, and had to do major cropping for these images.
For the lizard picture, I pressed lens against glass for stability. With clean glass, a still lizard or snake, and a gentle trigger finger, it is possible to get great shots through glass. Note that this is an HDR photo, with three images combined to optimized exposure.
If you drive, prepare to pay $20 for parking or join the Friends of the National Zoo (FONZ) for $40/$55 (live outside DC area / live in DC area) and have free parking for a year. Also be prepared to walk. Lots of whiny kids when we went.