1) Get rid of anything broken! Unless you're that rare parent that is actually going to fix it. If it's just another plastic toy, let it go!
2) Select only a few to keep in the room and box up the rest to cycle them through later. Put the box in the garage and out of sight.
3) Bring nature inside! Children love building with real materials. Have a variety of baskets filled with rocks, nuts, acorns, seashells, pine cones, sticks etc.
4) Have a cozy corner draped in material to create a little reading or resting spot. I loved building tents in my bedroom when I was little. It's so nice for them to have a special spot to just be.
5) Make it smell good! Lavender oil creates a warm, relaxing atmosphere in the home. A few drops on their pillow is just perfect.
You'll love how soothing the space feels when you're done and so will your child.