Old Halloween costumes make great dress-up clothes for kids... and if you have enough, or their friends have a few lying around have a costume Birthday party is a great way to to squeeze some extra use out of these, even if their birthday is in April!
Have a super hero party or a princess party and everyone wears their old costumes or if you have enough let them get dressed up there. You can make super hero badges or princess wands or jewelry as your party activities.
For cake get a more generalized image rather than a specific character so all the little ones feel important even if it is the birthday kids big day. Nobody wants hurt feelings and tears.
Let your imagination run wild and save a few bucks:)