Welcome aboard, Denise! Congratulations to your daughter on her college graduation! What a reward that she gets to now set sail with Disney Cruise Line! I wish I got to do that when I graduated!
You are welcome to bring along your own decorations and you can hang those up yourself at any time. However, just note that Disney Cruise Line does not allow adhesives of any kind in the staterooms, on the walls or doors. Many guests use magnets to decorate their doors or hang decorations in the room. So, toss some magnetic clips in your luggage to help put up your decorations.
There is a Bon Voyage Celebration decoration package available on the
Gifts and Amenities page of Disney Cruise Line's website, that the stateroom host would set up. However, I understand your concern about the price, so feel free to bring your own if that package isn't right for your family! Many guests will bring personalized magnets to put on the outside of their stateroom doors in the hallway to make their stateroom door a little more magical. Do an online search for "Disney Cruise Line graduation magnet designs" and I bet you'll find some designs that others have shared for guests to use. You can purchase magnetic printer paper that you run through your own printer with your design, cut out and pop it on the door! Easy as can be! Other guests like to print on cardstock, laminate and then put a magnet on back.
Have fun planning all the surprises, Denise!