The couple, who announced their engagement in December 2010, will marry in a lavish ceremony held at the Playboy Mansion next month.
Talking to US Weekly, the Playboy founder confessed that Harris suggested their 300-strong guestlist make donations to selected charities rather than purchasing items for the pair.
"It was actually Crystal's suggestion and I thought it was a great idea," he said. "The point of it is, we don't need gifts. A lot of people are in need and the notion of giving to charity instead of giving gifts to us just made sense."
The charities that guests can choose from include St Jude's Children's Research Hospital, anti-child prostitution organisation Children of the Night and charity: water, which aims to provide clean drinking water to poor regions worldwide.
It was recently reported that Harris will wear a pink wedding gown at the ceremony.