Wear your clothes backwards.

Greet your guests at the door, and say good-bye.

Walk backwards.

Play a game:

Write messages backwards.
First one to figure it out wins.

Tell knock-knock jokes backwards:

Give the answers first.
Have your friends make up the questions or joke.

Serve a meal backwards:

You guessed it, dessert first!

Make up a secret backward language:

Add "ay" to every word.
Example "funay" "partyay".

Read a book from back to front:

Do you know what will happen next?