Perform taps on the pillow. Taps are single hits of the drumstick so the pattern should be: left, right, left, right, left, right. Play this as fast you want.
Give the sticks to your friend and have them do the same thing.
Change the beat up a bit by performing taps with accents. Taps with accents involve playing one tap harder than the rest in a set. The pattern for these can be grouped in fours or, if you really want to challenge your friend, threes or fives. The pattern for accented taps with fours will be LEFT right left right LEFT right left right LEFT right left right (the capitalized words are accented).
Give the sticks to your friend and have them match your pattern. Keep going back and forth until one of you can no longer match the beat. Then start over.