When I make a full attack, can I choose to not activate the benefit of the Power Attack feat for the first attack, and then activate it on subsequent attacks? In other words, can I make an attack choosing to not Power Attack, and then activate Power Attack to have all subsequent attacks be power attacks?
The Power Attack feat says:
You can choose to take a –1 penalty on all melee attack rolls and combat maneuver checks to gain a +2 bonus on all melee damage rolls. This bonus to damage is increased by half (+50%) if you are making an attack with a two-handed weapon, a one handed weapon using two hands, or a primary natural weapon that adds 1-1/2 times your Strength modifier on damage rolls. This bonus to damage is halved (–50%) if you are making an attack with an off-hand weapon or secondary natural weapon.
You must choose to use this feat before making an attack roll, and its effects last until your next turn.
It doesn't specify you have to choose to use the feat before your first attack roll - just "an attack roll" in general.