[Canonman]

We're heading to Yellowstone in May as well. Here's a sample of what's on our list:
Hayden Valley is close to Fishing Bridge and probably the best place to see wildlife due to the lush wetlands along the Yellowstone River. Don't miss Grand Canyon of the Yellowstone especially Artist's Point. For wolves and pronghorns I'd recommend Lamar Valley.
For elk the Albright Visitors center is the place! While there Mammoth Hot Springs is a must see.
The Geyser Basin is a given but in the evening stop by the Madison Information center. Elk and Bison can be seen grazing all along the Madison River.
An easy evening trip from Fishing Bridge is to drive up to Lake Butte Overlook. Some of the most beautiful sunsets I've seen were from Lake Butte.