you gotta go see Old Faithful in the center of the park
and a must-see is the Mammoth Hot Springs by the north entrance - walk around the colorful hot springs (hold your nose!) and there are some nice short hikes up into the hills
the drive along the north end of the park is usually less crowded (Mammoth Hot Springs toward Tower-Roosevelt and beyond). This is where we saw bison fighting on the road
Check out the Roosevelt arch at the north entrance (just outside Gardiner, MT)