It is a very creative idea to invite all your girlfriend's close friends to hold a birthday party for her. It can be seen that you are a thoughtful boyfriend. Since you have decided to give her a surprise, do it to the end. The gift must not be done in a hurry. The boyfriend's gift must be the highlight on his birthday.
It is recommended to customize a DIY chocolate for your girlfriend, write the blessing language you want to write, and then suggest giving her a ring. Of course, it is not a simple gift. You can put it in a special wooden small house model with a mechanism, so that she can slowly find a way to open it to reveal the ring.
You can also give her a girlfriend's imperial edict on that day, customize the fun words you think of to express your love on bamboo slips, and announce it at the banquet, for example, words like "I want you to be happy", which are enough to surprise and move you, hehe. I hope it will help your plan.