Are you going to theme it? Home or venue? Alcohol or not? Games?
BBQ’s are good. I’m thinking of doing a ‘Ladies a plate’ party soon, everyone dresses up 50-60’s style and we have a Devonshire tea kind of thing. There’s too many options.
What are you and your friends interested in, you could theme it around mutual likes.
Pick a letter of the alphabet and everyone comes dressed as something starting with that letter.
Clothes swap party. I’m probably not being very helpful but I hope whatever you decide you have a great celebration. Happy Birthday