Honestly, with the fact that you won't see spellcasters perform magic outside the Realm (unless it's one of your played Sims), there's definitely a way to make the presence subtle or pretty much non-existent outside of decorations.
With the contents of the build/buy and even CAS, it could be something like honeypooh1267 said: maybe your family inherits a home in Glimmerbrook from a distant relative. When they check it out, there are things they didn't expect: a massive cauldron in the shed that they first assume was for cookouts, an old finely carved crib that they take a liking to, a herb garden with plants they haven't seen before. And maybe one of their teens starts poking at the bookshelves and finds the books there intriguing so they go wandering around the forest and stumble upon the portal.
Or maybe your Sims befriend the families living in Glimmerbrook and that's how they find themselves exploring that world and what lies beyond it.
Or (if you get one of the potions from the build/buy catalog) a Sim accidentally drinks a potion and they need to find a spellcaster to undo its effects.
:D I hope you find a way to make this pack work for you, whether it's with or without the magic gameplay!
