Super cute idea for a playroom! DIY magnet board - 1 sheet of galvanized metal (comes in many different sizes in the plumbing section) and wall trim or frame. Cover metal with fabric. Would also be great for displaying pics you get from relatives/friends.
Crayon hot rocks: If you have not done this yet, you need to. Simply warm up the rocks in the oven (I did 220 for 10 minutes), then put them on a stack of newspapers (or something else you don't care if it gets crayon in it) and let the little ones go to town. Just remind them it is hot.