Drew Barrymore, Cameron Diaz and Reese Witherspoon Do Girls' Weekend in Mexico

Caught in the Act

By Aili Nahas

03/08/2013 at 10:20 AM EST

What's more relaxing for a couple of new moms than a few days on the beach?

Reese Witherspoon, 36, and Drew Barrymore, 38, left babies Tennessee and Olive, both 5 months, at home, and joined pal Cameron Diaz, 40, and five other friends for fitness retreat Bikini Boot Camp at the Amansala Resort in Tulum earlier this month.

"It was a girls trip," says a fellow guest. "They all seemed like great friends who were there to relax and have a good time."

A good time, and a little hard work too.

Drew Barrymore

FameFlynet

"They participated in tons of fitness classes," says the source. "None of them wore makeup they ! were fresh-faced and natural."

During free time, the girls lounged on the beach. "Drew and Reese looked great for just having given birth," says the onlooker. "And Cameron is so ripped! She's incredibly fit."

At night, the girls bonded over meals of fresh fish and vegetable soup. "Drew was showing off pictures of her baby on her phone. Everyone was in great spirits the whole time."