There are only a few times where the only condition is incapacitated
The Arcane Archer's Banishing Arrow states:
While banished in this way, the target's speed is 0, and it is incapacitated.
Both the banishing smite and banishment spells state:
While there, the target is incapacitated.
The symbol spell states:
Pain. Each target must make a Constitution saving throw and becomes incapacitated with excruciating pain for 1 minute on a failed save.
The wind walk spell states:
Reverting takes 1 minute, during which time a creature is incapacitated and can't move.
The Short Term Madness table in the DMG states:
21-30 The character becomes incapacitated and spends the Duration screaming, laughing, or weeping.
The Wild Magic Sorcerer's Wild Magic Surge tables states:
41-42 You turn into a potted plant until the start of your next turn. While a plant, you are incapacitated and have vulnerability to all damage. If you drop to 0 hit points, your pot breaks, and your form reverts.
And the Unearthed Arcana Raven Queen Warlock's Sentinel Raven feature states:
While perched on your shoulder, the raven can't be targeted by any attack or other harmful effect; only you can cast spells on it; it can't take damage; and it is incapacitated.
Most of these either make the target go to another plane of existence or they affect the player and not a target. The only one that could be used offensively is the symbol spell's Pain option.
There aren't even very many ways to apply the incapacitated condition with other conditions, which, as user @thedarkwanderer pointed out, would inflict only the incapacitated condition if a monster were immune to the other condition.
The Tasha's hideous laughter spell which inflicts the prone condition:
The target must succeed on a Wisdom saving throw or fall prone, becoming incapacitated and unable to stand up for the duration.
And finally the feign death spell which inflicts the blinded condition:
The target is blinded and incapacitated, and its speed drops to 0.