For my daughter's 3rd birthday I made invitations out of black card stock. I cut them to look like Mickey's head and folded them over. I used a silver sharpee to write on the invitations. On top, I wrote something like "It's a Mouskeparty!" or something like that. On the inside I wrote the time, date and address. I stuck them in black envelopes and sealed them with a mickey sticker.
My son is turning 3 next month and wants a Mickey party, so I'll be reading this thread to see if I can find some new good ideas for his party invitations.
I was thinking about printing white pieces of paper with a mickey clip art using Disney font for the party details, then gluing the white paper onto a piece of card stock (maybe red?). Then, using a Mickey hole punch, either punch Mickey heads around the edges, or use the hole punch to make Mickey head cutouts in the other MMCH colors, and glue them around the edges.