The only fact that you are 15 and so caring for your mother speaks so high of you...god bless u . as far as surprise is concerned,,,,,,it is not the surprise that you are looking in your mothers eyes , it is the happiness in her eyes what you are looking for.
do what ever you plan to do....cake, eat, sing, dance what ever with in your own budget
but at the end of the party just sit down on your knees and tell your mom that you are so lucky to have her as your mother and tell her how much you love her.....not from the tounge but from the heart,,,,and trust me,,,,,your mom will understand when you are telling a truth or a lie....and more than any surprise and any party your mom will remember this incident through out and at the end of the day when she hugs u i am sure you got your return gift too.....
i dont want you to rate my answer as the best answer but yes a warm hug from my side too and wish her many more wonderful years ahead
lots of lov