ANON'S GUIDE TO SECRET HOBBYING
First, get yourself an actual hobby. You should especially target a given area that your SO shows no interest in. In fact, make it something that bores or abhors her. Transfer that to activity on eBay. Get a Paypal account with a ATM card and credit card for them, and a savings account dedicated solely to that. Make sure you have plenty of artifacts to serve as proof props laying around the house (ie. coins, stamps, comic books, beer cans, trains, etc.). Keep your account stuff stored in a box mixed in with all the other "boring" paperwork of your "hobby." Heck she will probably even suggest you get a cabinet or freestanding closet to keep all that "shit" from being scattered about the house. If the hobby includes valuables, like coins, now you have a reason to keep a lock safe.
PLUS! Now you have:
1) A beard (alibi) for occasional out of town trips with hotel stays (hobby conventions).
2) A "secret" banking account.
3) A "secret" credit card.
4) Two ATM cards (comes in handy for larger withdraws for HDH and duos)
5) A wire transfer service.
6) A cash generator (if you are smart you can resell and make a profit).
7) A way to to hide any transaction, or excuse any discovered transaction with a reasonable excuse that will make it even easier because her eyes glaze over whenever you start talking about your Pez Dispenser collection and the convention in Louisville, Kentucky next weekend!