21st birthday party decoration ideas include ^{1 2 3}:

1. Use a 21st birthday banner, birthday balloons, table decorations, garlands, and string lighting.
2. Consider smaller elements like matching paper straws, party favor bags, napkins, and cupcake toppers.
3. Create a stylish and elegant atmosphere with black and gold decorations.
4. Use balloon arches and garlands for a festive look.
5. Set up photo backdrops with images of the birthday person.