Qwak is a GREAT recommendation, awesome game, very simple to pick up and play, great for quick sessions (But, again, when you get good at it, you may have longer sessions
).
Bomb Jack is also a very good recommendation (The new Beer Edition please, not the shody oficial port), it's very simple to play, excellent "pick up and play" gameplay...... though it's far of being an easy game. Which means gaming sessions will be very quickly
(Either that or I *really* suck at Bomb Jack, my tries on it on the arcade version usually end up very quickly too
)