You can reward yourself for completing a seemingly arbitrary task. Every morning after completing my Anki flashcards, I force myself to lean back, smile, and feel proud for completing an exercise in discipline, even though I don't _really_ care about the cards. This alone is enough to maintain the habit.
This is enough because you have self-directed control remaining - you're pushing from lack of reward towards short term unrelated reward, but this deception long term causes struggle with self-efficacy, especially when the small short term reward loses its potency.