My kids are older (8 and 11) but here's what they play with most of the time:
Indoors:
Dolls or action figures
Cardboard boxes and other empty containers which they use to build stuff for the dolls/action figures
Fabric scraps, yarn and buttons which they use to make clothes for action figures and dolls
Legos
Drawing/Painting/Clay
Each other (rolling/wrestling around/other physical play)
Outdoors:
Sticks, dirt, old hoola hoops, the neighbors' dogs, bikes and other ride-on cars, water.
I supply plenty of tape, glue, markers and other materials.
