Inside Gwen Stefani's Birthday Bash
10/07/2008 AT 12:30 PM EDT
Stefani spent most of the night on an outdoor dancefloor at the bash hosted by husband Gavin Rossdale and attended by about 100 guests, including her No Doubt bandmates, Ashton Kutcher and Demi Moore, newlyweds Ellen DeGeneres and Portia De Rossi, Nicole Richie and Joel Madden, Christina Aguilera, Duran Duran's John Taylor and Stefani's own Harajuku Girls.
"The deejay played great music from the '80s and all her favorites – reggae, songs by The Specials, Fishbone, The Police," says a guest. "She danced all night and went from person to person just being the birthday girl."
Stefani, who gave birth to son Zuma on Aug. 21 (his brother Kingston is 2), wore a black party dress with her blonde hair down and wavy. "You'd never guess she'd had baby 6 weeks ago," says the guest. "She looks great."
Guests dined on mini BLT sandwiches and pizzas and sang "Happy Birthday" to Stefani, who erupted into laughter when she was presented with a birthday cake decorated with pink candles.
"She just looks really happy and so good," says the guest. "She and Gavin were so sweet together."
For more on Stefani's birthday party, including details on her one-of-a-kind birthday cake, pick up PEOPLE on stands Friday.