Mobiles are a great idea as you get to use all your crafty creations to hang onto a mobile!

Actually, your post reminds me of a craft idea my then 20 month old came up with all by himself:
Making cheery, aqua-coloured nappy fillings. It takes a while, but here is how it goes:
Give your toddler some aqua coloured play doh, let them play with it, then eat it. Wait til the next morrning and be prepared for a surprise when you open their nappy