Whether Mickey Mouse or another Disney character, the book titled the Disney Party Handbook is a great resource for themes, food, games, and decorations.
For decorations, use red and black helium balloons on the ceiling. Have polka dot table coverings in red, white, or black. Used stuffed characters as centerpiece.
For party invitations check out www.birthdayinabox.com or http://disney-stationary.com/greeting-cards/birthday-invitations.php
You can get special Disney M&Ms at http://www.mymms.com/Disney/selling_page.asp, but my favorite Disney treat are Disney decorated caramel apples. Check out photo at
http://hereandthere.us/wp-content/uploads/2009/07/mickeymousecaramelapple.jpg
