That '70s Show - Season 5 Episode 15
When the Levee Breaks

When Bob goes out of town, Eric and Donna decide to "practice" being married at her house. The gang is excited because they think this is a chance to party all weekend at the Pinciottis place, until Eric and Donna are forced to set them straight. Meanwhile, Kitty goes to the jewelers to pick up a ring she thinks Red got her as a surprise. But she discovers the real surprise: its actually an engagement ring for Donna.