Stella had a mobile on her crib, but we never really used it. When it was time to decorate Lu's nursery I didn't even bother with one. Then... I saw one on Pinterest (of course) and knew I NEEDED to make her one. Of course, you can't do something for one and not the other little one these days so I made one for each of the girls' rooms. They're just simple little pinwheel mobiles. Here's a shot from below:
The third year these two have been able to go on the Giant Slide together - they both love it!