I think there should be one fight with a champion of good. As mentioned above, it is somewhat unsatisfying, but I think there should be at least one moment where the PCs face someone doing their duty, and they realize they had to do theirs as well in order to proceed. The opponent warns them off, first sternly, then gently, then pleadingly, and finally attacks, without hesitation and without using nonlethal attacks (unless advantageous). A hound archon would be good for this, but you could also use a LG elemental with class levels, some kind of angel, a ghost, or a mortal paladin with divine rank 0. If the opponent is defeated, it has still fulfilled its service, and if shown mercy, will visit no further harm on its honorable opponent. The "good guys forced to fight" motif has been good for a scene in many films. But this is not something you want to overdo.
Traps. Magical traps that trigger summon monster in conjunction with other effects would be effective, and would introduce celestial opponents to fight that would not have to be murdered (only defeated). Traps that fire off smite evil and dictum would be excellent for raising the pulse of LN or NG party members.
One room should have a mirror of opposition. Not only is this a great deterrent, it leads to a great battle, thematically, for good aligned characters. The mirror would be maintained by some entity, possibly a protectar, angel, hound archon, and so forth.
Some celestial pixies... they demand the PCs perform some tests of character and worthiness before proceeding.
Constructs are a good idea. Giant stone statues of interesting figures, enchanted with permanent magic mouth, tell the PCs that it is forbidden to enter.
Tests of courage... narrow beams over vast pits, that kind of thing. Putting a spear trap in the middle of a narrow beam would catch victmis flat-footed if they didn't have 5 or more ranks of Balance, and taking damage could cause victims to fall.