Across the whole back wall I would build a deep shelf with an open bottom at table height. For now it will be good for parking ride on toys under and setting larger playsets like Little People, Barbie, and Hot Wheels. You can add a couple of baskets to catch miscellaneous small toys. I like to group them by people/animals, cars, etc.

Later, it will serve as the perfect place to stand and build with Lego, and finally when they are Middle/High School age you can create a study area there.

Under the window you can place a bookshelf, and use a sofa floating in the room facing the toy shelf to divide the play area from your work space.