Create cards with the time, date, place and other party details on the outside cover, and the "you're invited to a backward bash" on the inside.
Ask your guests to show up with their clothes on backwards.
Hang balloons from the ceiling with string.
Erect a "yadhtriB yppaH" sign.
Place silverware and dishes under the table.
Food & drink
Serve cake and ice cream first (kids will love you for it, even if the parents don’t), followed by your main dishes, and finally appetizers.
Pour hot chocolate for dessert, and cold drinks later.
cookie and cone birthday clowns
are perfectly quirky.
Fun & Games
Play "cold" potato instead of "hot" potato. It's the same rules, except you pass the potato when there's no music, and the person holding it when the music starts is out.
Also, try "pin the head on the donkey", and "seek and hide," where one person hides and the rest of the gang must find them.