Sand Point Picnic Area
This is probably the closest you can get to a beach experience while in the park. A paved loop road takes you around this large picnic area that has 15+ well-shaded tables and two restrooms. At the end of the looped road is a trail that leads you through some lush forest area, over a small stream, and toward the lake – only taking you about 3 minutes to walk. Once there you have access to an extensive sandy beach with a sandbar that runs both directions for as far as you can see before curving out of sight. A nice place to enjoy walking and relaxing.
Sand Point Picnic Area has been mentioned in the related articles:
"Best Places to Go and Things to do with Kids in Each of Yellowstone's 14 Road Sections"
"Best 9 Picnic Areas in Yellowstone- For the Whole Family"