How do I word a card that says we'd like our guests to donate to charity instead of giving us gifts?

You may wish to consider something similar to the following:

“(Name) and (Name) have been fortunate

to benefit in life and in love.

If you were considering a gift,

they would be happy to accept a donation

to your favorite charity on their behalf.”

You are not supposed to say or even imply that a gift is expected in any way. This includes mentioning where you are registered. Instead, you are supposed to tell your best friend to spread the word where you're registered, or what you want, or whether you'd prefer to have donations made to Aunt Tilly's Home for Unwed Cats. When you mention a registry, you look like you are telling your guests that they aren't welcome unless they bring a gift, even if you state that you want donations rather than gifts.

This question often comes up in etiquette/advice columns, and the answer is always the same...you can't flat out state that you want a gift. The ONLY time it's acceptable to state something like this is a funeral notice, where you will sometimes see "In lieu of flowers, please donate to <charity>". This is because the grieving family has enough on their hands to take care of questions like this.

In lue of gifts please donate to the charity of your choice

just say it