Decorations
There are a couple of ways to organize a charitable birthday party. One is to plan a party at your home with a traditional theme that highlights your charity. For example, if you are helping your local humane society, you can have a puppy-themed party with invitations, decorations, activities, menu, and favors that match the theme, and you can request that your guests bring an item for the animals.
Alternatively, you can host your party at the charity you have chosen if this is allowed. Call the organization well in advance of your party and let them know your intention to raise money or donate goods. Ask if they have room to host the party at their site. If so, see if you can decorate an area for the party. If not, given the charitable nature of the party, just a few decorations will do nicely.