Since your intended audience is co-workers, I'd keep it clean. So no nursing tanks cause I'm not sure guys would want to know that. I'd keep it simple too. Since anything you get is 1 less thing that you'd need to buy, register for things you would buy. Bigger gifts could be a group gift (but then I wouldn't think they'd want you to register).
Go for things like crib sheets, diapers (but not too many; who knows what you'll really like), burp rags, bath toys, exersaucer, bouncy seats, bath tub, washcloths, some clothes in varying sizes (remember the season), bottles, pacifiers, baby blankets.
I'd keep items at less than about $50.