My wife is used to seeing kits/snares/cymbals/gear galore come into and out of the house at a rapid pace. I buy entry-level kits that I find on craigslist for great deals and sell them to students (I live in a poorer neighborhood where the kids can't afford even the $250 for a basic shell pack, let alone any cymbals to add on top of that). She doesn't keep tabs anymore about what's coming and staying or what's going. That works for me! For all she knows, that "new" 1959 Slingerland kit that just showed up in our basement is for a student.
The cases idea has the potential to work, or any "out of sight" location. Heck, I could FILL my studio with drums if I wanted, as my wife NEVER goes in there. Do you have a closet that never gets opened? A dark corner in a basement? An attic?
Basically, it boils down to this: unless they're into it too, you're going to have a very hard time explaining to your S.O. why you "need" that new snare/kit/cymbal/whatever. Actually, none of us really "needs" this extra stuff, but as curious human beings, we desire novelty and variety, and as musicians, we desire inspiration in the form of new looks/sounds, and as consumers of products, we simply desire more stuff. When redundant instruments come in the form of expensive objects that take up space and serve no perceived needed purpose to our loved ones, it doesn't make sense to them and will seem unnecessary and frivolous. And, quite honestly...IT IS!!! BUT, it's our passion and hobby (and to some of us, our lifestyle and livelihood), and we can't expect others to understand all of our quirks and obsessions.
...and so, THAT is why the answers to this question will mostly be sneaky ones. If you can manage to be upfront with your girl, and she's cool with it, then godspeed! Go forth and collect! If not, then you'll have to come up with other, "less upfront" measures like the rest of the gear-heads you'll talk with here...