A carpet in a soft grass green color or rosy pink that compliments the walls and bedding is a nice touch. A neutral color also blends in well and won't detract from wall art and other decor.
If you wish, you can keep his bedding the same for a while, to give him the comforting feeling of some sort of continuity in the transition. Or you can have brand new custom bedding designed to match the theme of his big boy bed. Which option is best depends really on the child.
Attach a mobile to the crib rail when your baby is an infant, but remove the mobile when your baby can sit up on his own.
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