Eyewitnesses tell Us Weekly that the couple was spotted celebrating a joint bachelor/bachelorette bash -- sans any x-rated entertainment -- with a night on the town on Sunday.
The celebration reportedly began at Eveleigh restaurant on Sunset Boulevard were the 33-year-old fashion designer and her fiance dined on small plates and sipped on cocktails along with their friends.
"[Simpson] couldn't have looked happier," a source said of the evening.
The group eventually made their way to Warwick nightclub, where they were photographed leaving early in the morning the next day.
Simpson and the former NFL star have been engaged since November 2010 and are expected to tie the knot in July. Two have two children together, Maxwell, 2, and Ace, 10 months.