Singing:

Spoiler: show

Rub Pleo under his/her chin

.
Sitting:

Spoiler: show

Hold back and chin sensors and help to sit when he/she pulls their feet together.

Tadaa (balancing):

Spoiler: show

Hold two opposite leg sensors.

Default Watchdog mode (no sd card required):

Spoiler: show

Hold both front leg sensors, the rear left leg sensor and the shoulder (?) sensor at the same time. (Takes perseverance and practice )
Raise foot/shake hands: touch appropriate leg sensor.

High step mode:

Spoiler: show

Hold two front leg sensors and Pleo button until Pleo honks or starts to walk.

Sleep:

Spoiler: show

Hold Pleo's back sensor until he goes into sleep position.

The wave:

Spoiler: show

Rub slowly from tip of nose all the way down the back. (Pleo should respond to each sensor in turn)

Holding Pleo against shoulder:

Spoiler: show

Rub down Pleo's back to calm him/her and he/she will stretch legs out and nuzzle into you.

Tickling Pleo:

Spoiler: show

Turn Pleo upside down (yes he will squirm at first) and rapidly tap random foot sensors until giggling occurs (or Pleo gets really mad )

Tug of War:

Spoiler: show

Have Pleo in a good mood and not hungry and offer him/her the , they will open their mouth for you to place the in and play tug of war

Detect the edge:

Spoiler: show

(Not really a trick but great to show people) Place Pleo in a well lit area on a small table etc, ask Pleo to walk forwards.

Explore mode:

Spoiler: show

Hold Pleo's to front leg sensors and he/she will go off and explore.

Other funny tricks:

Spoiler: show

Soccer Pleo: Place a small ball (small cat toys work great) in front of Pleo and get him/her to walk forwards, they will dribble and kick/nudge the ball.
Demolition Pleo: Set up a tower of light objects (polystyrene cups work well) get Pleo to walk towards them.
Stinky feet: Great for when guests are over, point Pleo at their feet and get them to walk towards the visitor, Pleo will sniff their feet, absolutely hilarious if Pleo growls or coughs
A-maze-ing Pleo: Set up a small maze of boxes and time how long it takes your Pleo to get through (Make sure you have a whole day free and a good book )