The most significant 'side activity' in Castlevania III is experiencing all possible character combinations. Since you can only recruit two out of the three potential allies (Sypha, Grant, Alucard) in a single playthrough, you'll need at least two full playthroughs to see all of them. Each ally offers a drastically different gameplay experience.
- Path to Grant: After Stage 1, take the upper path to the Forest of Silence. Defeat/recruit Grant.
- Path to Sypha: After Stage 1, take the lower path to the Abandoned Mine. Defeat/recruit Sypha.
- Path to Alucard: Alucard is always encountered in Stage 3, regardless of whether you recruited Grant or Sypha. If you went through the Forest of Silence, you'll find him on the Ghost Ship. If you went through the Abandoned Mine, you'll find him in the Sunken City.
Experiment with different combinations. For example, a Trevor/Sypha run offers powerful magic, while a Trevor/Grant run provides unparalleled mobility. A Trevor/Alucard run gives you flight and strong projectiles. Each combination requires different strategies for stages and bosses, adding immense replayability to the game.