* Sí, hago ejercicio hoy. (Yes, I work out today.) - If you do plan to exercise.
* No, no hago ejercicio hoy. (No, I don't work out today.) - If you don't plan to exercise.
* Sí, pienso hacer ejercicio hoy. (Yes, I plan to work out today.) - A slightly more nuanced answer implying intention rather than certainty.
* No, no pienso hacer ejercicio hoy. (No, I don't plan to work out today.) - Similar to the above, but indicating a lack of intention.
* Tal vez haga ejercicio hoy. (Maybe I'll work out today.) - If you're unsure.
Choose the option that best reflects your plans for the day.