As Newf said, throw it in a drawer. It's only a couple of days, it isn't like it really matters. CPU's aren't that delicate. The only things to really be careful of is to make sure nothing gets on the pins and to make sure you ground yourself to something before you touch the CPU and you'll be fine. Even that is taking some pretty hefty precautions.
Just for example, I accidentally left a processor in a cardb.oard box with no bagging or anything outside and it rained.. the box was filled with water and the processor was soaked. I let it dry out for a while, cleaned the pins with isopropyl alcohol, and it worked fine. While I'm not saying that's how you should treat your processor, don't freak out about storing it. Think about what it goes through while in the actual computer: rapid temperature changes, dust all over the place, and many pounds of pressure placed on it by the heatsink
In short, just throw it in a drawer and don't worry about it.
edit in response to your new post: if there is paste on it, you're going to want to clean that off with 90%+ isopropyl alcohol.. you can't reuse it. You'll also want to clean off your heatsink and apply a new batch when you get the new motherboard.