With a visit to the Ruins of St. Paul, it's hard to miss visiting Mount Fortress as it's right next door.
The only time I would recommend against visiting is if you have a young child with you, or have an elderly. There's two ways to get up. You can walk up the steep steps adjacent to St. Paul on the right side, or through the back up the hill (which is an easier but longer route).
If you plan to visit the Macau Museum, allocate sufficient time (at least ~1 hour) as the Museum closes at 6pm (last entry at 5:30p).
If you enjoy Museums, give the Macau Museum a try! For 15 MOP (~$2 usd for adults), it's a great way to spend an afternoon from the humid weather. If you are lucky, sometimes they have free admission days too. I loved all of the interactive displays and everything is signed in English as well.