Yellowstone is full of geo-thermal activity.  The most well known geyser is Old Faithful.  There are several other large geyser networks located throughout the park.  There you will find many hot springs and pools.  One of my favorite geysers is rainbow pool.

    There are many rivers in the park.  The Yellowstone River is probably the most photgraphed.  There are several smaller ones that can be found by driving around the park. 

    You could spend an entire day photographing the landscape just from the road, not including any of the several hundred hikes in the park.  Much of the landscapes in this park have been shaped from the geo-thermal activity.

    There are several waterfalls in the park.  The most famous one would be Lower Falls on the Yellowstone River.  I was also able to get photos Gibbon Falls along the Gibbon River near the Madison junction.