Do yourself a favor and spend the $45 for a can of Bear Pepper Spray.
This is the best rec I can think of and it's available in almost every park store. But I am a little sensitive on the subject.
Realy though, Yellowstone Is a wonderful place, that goes without saying. Old Faithful Village is very nice. Dont miss the thermal features just down the road from OF about 1/2 Mile. I forget the name but you can't miss it. I think it's where "Morning Glory" pool is.
Check out the sunset views from the North lodge, OF is awsome that time of day from that point.
Keep you eyes open for bison. They are quiet and you can turn a corner around a building and be smack next to one if your not aware.
Keep your distance from these creatures. They are awsome.
Where are you camping while in the park? I can tell you more of what we did while we were there if you would like.
Sorry to sound finatical about the Bear spray, It's just I found it to be the BEST thing I bought in the park.
But thats a story for another day.