Yes you can. You can declare a stunning fist attempt before making an unarmed attack, and you can make a trip manuever in place of an attack. Either of those are possible for an AoO.
Yup.
And if you had Greater Trip (and Combat Reflexes with an attack of opportunity remaining for the round) you could attempt to trip the target as an AoO, and if successful make a stunning attempt on the AoO provoked by the trip by way of Greater Trip. Thus, you could potentially leave the opponent prone and stunned for a round (or whatever other effects your monk can do with a stunning attempt).