Welcome to the forum and grats on the upcoming new family member. I'm expecting my first (and probably only) grandchild in September, but I do a lot of crib sized donation projects.
I recently did a project that used a bag of scrap fabrics, including some of the "new" greys. I thought they were fun to work with and more soothing than black and white. With the fabrics I had, which sound sort of like what you have, I think any geometric design would be my first choice. I can imagine a grey and white log cabin that would come out lovely -- of course there is always that big step between what I think something would look like and how it actually looks!
Polka dots are very versatile, and while I would first go geometric design, lots of things you can do with a grey and white polka dot, like an appliqued Elephant or Teddy Bear or other pictorial block.