Do a Minnie Mouse theme, Her red and white polka dot dress and her black ears would look great in a black and white room. The cake would be fairly easy to create (one large circle for the face, two smaller circles for the ears) and she will just have been to Disney World. Here are a couple of ideas for decorations and games from a website:
Decorations
Decorate the Mickey and Minnie Mouse party in Red, White and Black with a little yellow thrown in. Use Red tablecloths and solid white plates (Minnie Mouse's polka dot dress), black cups and yellow utensils. Use helium balloons tied in bunches for the party table or tie on the back of the chairs. Black, red and yellow streamers are always a fun touch.
Games
Mouse Ear Treasure Hunt
Object of the game is to find the hidden mouse ears - the special red ears can be turned in for a prize.
Cut mouse shaped ears out of posterboard about 3 - 4" wide. Cut most of the ears out of black poster board or cardstock, but also cut 3 or 4 (more depending on how many special prizes you want to give out) from red poster board or cardstock. Buy several small prizes and number them. On the back of the red mouse ears place a number that corresponds with one of the prizes. Hide the mouse ears all over the backyard, park or party room. Send the children off to look for the hidden mouse ears. If they find a red one they get the prize that corresponds to the number on the back.
Variations:
Hide only red mouse ears along with candy and treats.
If you are having a piñata you can enclose the red ears with the party candy.
Pin the Tail on the Mouse
Buy a large poster or draw a picture of Minnie or Mickey Mouse on a large posterboard. Trace their tail from the poster and make several tails - one for each party guest. Write their names on the back of their tail and attach a piece of double stick tape. Hang the poster on a wall and line the children up opposite it. Blindfold them one at a time and see who can place their tail closest to the tail on the poster. The closest to the original tail wins.