I recently did a crochet edged fleece blanket for a shower gift and I absolutely loved the way it turned out. Lots of cute neutral or primary colored fleece out there in tons of themes from rainbow to Pooh.http://www.bernat.com/home/babyroom.html
has a mobile, bears, blocks, booties that could all be made in any of the awesome gender neutral colors everyone else has suggested.
If you have SnB: Happy Hooker..how about the Seija set in some nice neutral colors. Sock monkey set from that book as suggested by Maltiriel is another good choice. I've made both and they work up quick.
Good luck!! I'm sure Mom and Baby will be thrilled with whatever you choose.