I tried that once, the results were not very pretty!
Seriously, do not hide it - discuss it. My wife wants to buy a new television, so we discussed it. She knows I would not have been thrilled to come home and find a new TV in the living room. Big surprises are not usually welcome, and even little surprises can cause disagreements. For example, we had one artificial Christmas tree, it's perfectly fine if you like that sort of thing. I mentioned getting a real tree this year. I come home the next day and there's a NEW artificial tree in the room. We now have TWO artificial trees. I would have preferred talking about that in advance. What are we going to do with the other artificial tree? We also live in a one-bedroom apartment so space is limited.
When there are other more important things to think about (like raising a child or just paying bills on time), items which are basically luxuries must be put on hold. I mean, if this purchase wasn't going to be an issue, you would not be thinking about how to hide it, would you?