How's this for an imaginative invitation? Inflate a balloon. Pinch end shut but don't tie closed. Write invitation details on the balloon with a bright permanent marker. Make sure the ink is dry before you deflate. Place balloon in an envelope and mail one out to each guest. When your guests receive it, they'll have to blow it up to see what it says.
Note: It is suggested to use a pump for sanitary purposes.
Now I want to have a party.