When DS was little, I used to use the fitball too, but I moved it into the living area, so settled DS while talking to DD and then moved him to the bedroom once he was asleep.
What about stickers? I bought a whole heap from a cheap shop and a plain scrapbook pad, and DD would be happy sticking them down. Or something that they can sit at a table doing, so playdough, crayons, etc. I would often use the HAB too during the day for long sleeps, and so I'd take DD into the garden to run around, or if it was raining, we would walk up and down our hallway, which was very long, and I'd get DD to do big steps, little steps, side steps, backwards etc. That would entertain her for ages and DS liked the motion.
As DS got older, he became really hard to settle at night, and if DH wasn't home to help, I would often put a movie on for DD otherwise she would keep coming into DS's room and distract him.