I think the basics are that it depends on your timeline. If you plan to use the money soon (within, like, a year or so), then a savings account or something is just as good as anywhere because it's coming out too soon to matter. If you plan to use it in five years or so, then putting it into bonds will give it some growth without risking lost. If you plan to use it in ten years or so, then stocks or a mix would be fine provided you start to move it into bonds/savings closer to the time of purchase.
Since you only need 15k left, it sounds like you are pretty close, so stocks are out. I you're going to wait a few years, you might put it into bonds or something similar.