Just a word on timing, Yellowstone is one of the most popular national parks to visit, and in July and August the place is super packed. Expect long lines for parking and backups on the Loop Road.

Which brings me to my suggestion, Ive been during mid June (2nd and 3rd weeks) and the park has been pleasantly deserted. The reason for this is that summer comes late in the mountains. We still saw some snow in some locations, and the highs were only in the low 50s some days. However, I would strongly recommend that if you are going to do a summer trip to Yellowstone, you go in mid June.