There has been a lot of valuable information alread given in this thread, but I thought I'd add my two-cents worth! My husband and I loved Yellowstone when we visited a few years back (we were there the same time as Rita!) Some of the main attractions there are certainly must-do's:

Grand Canyon of the Yellowstone
Old Faithful area (Upper Geyser Basin)
Mammoth Hot Springs
Tower Falls
Lower Geyser Basin
Norris Geyser Basin

Try to stagger your geyser days with non-geothermal sites if you can, to keep it all fresh. And if you can, do get off the beaten track and explore some of the less-traveled areas and trails. One of our favorite memories was hiking the (Grand Canyon) South Rim Trail to Point Sublime and then doubling back to pick up the Clear Lake Trail loop. We saw tons of backwoods geothermal features (Clear Lake itself is a bubbling geothermal lake) and only saw three other hikers the entire time we were on the trail. We also saw evidence of recent bear activity. Always pay attention to the rangers as far where bears have been sighted and follow their guidelines related to bear safety!

Lamar and Hayden Valleys are both popular wildlife-sighting areas. Lamar is beautiful and expansive; if you have time, continue on past the valley and drive 212 alongside Soda Butte Creek for a while. There are some very picturesque spots and hiking trails. We did the Trout Lake trail, a steep but short hike to a breathtaking view of the lake.

Although Lamar is a favorite for some people because of the wildlife you see there, we actually preferred Hayden Valley. It is a much more intimate feeling, as far as the Yellowstone River running through it and the animals you will see around it. This area is famous for "bison jams", where large herds of bison will cross the road, causing long traffic backups. The only thing you can do is sit back and enjoy the view. There is nothing more humbling than a bison weighing one ton or more walking past your (closed) car window!

Which leads into another piece of advice. Guidelines state that onlookers must stay at least 100 yards away from bears or wolves and at least 25 yards away from other wildlife in Yellowstone. I cannot tell you how many times we saw people walk within a few yards of elk or bison that could have charged them at any moment. They were crazy stupid! Also, stay on the boardwalks and designated trails, especially in the geothermal areas. We went on one ranger-led tour where she recounted stories of people who had suffered burns by stepping off the trails and then breaking through the surface crust into scalding water below.

It's good that you are planning so far in advance, especially if you want to stay in one of the on-site park lodgings. We stayed three nights in a ranch cabin outside of the west entrance (near the town of West Yellowstone) and two nights in Lake Yellowstone Hotel (a very cool place!) From there we traveled down to the Grand Teton National Park and stayed three nights in Jackson Lake Lodge, our favorite lodging of the whole trip. The Grand Teton area has amazing scenery, prettier in some ways than Yellowstone, so if you have time to combine the two parks in your trip it would be worthwhile.