A friend of ours asked that instead of giving gifts for her birthday, she would rather prefer us to donate $ to charities on the invitation. How do we do this? Do we put a check in her card written out to the organization? Is it appropriate to also give a small gift to her as well?
If you are doing the donation "in lieu of gifts", then giving a gift also would be tacky. You probably won’t offend the friend, but you make make her or other guests feel uncomfortable if others did not bring gifts.
As far as the donation, you should make the donation to the organization yourself, with a notation that you are making the donation in honor of your friend’s birthday, rather than giving your friend the check. Giving her the check will make her party seem more like a fundraiser for the cahrity than a birthday party.
Sometimes, the charity may have a card stating something along the lines of "a donation has been made in your name". If they do not have a card, then specify which charity you danated to in a simple note inside your friend’s birthday card.
To make the donation special, make sure you donate to a charity that your friend supports. (Ex: Donating to a Christian church-based group would be inappropriate if your friend is Jewish)
Some charities seem to have a particular niche ("all proceeds will go to dwarf bisexual Eskimo bikers") If you are not sure of which charity she supports, be sure to choose something fairly neutral, like Habitat For Humanity, the United Way, or the American Cancer Society.