Swings and a crash mat. Obviously depending on DC age and safety abilities (ligature risk etc).
My kids are older now with only DD 8 playing with toys (although her teen and adult siblings will join in) but if I had to design a playroom now I’d think about:
Clear floor space to make Lego/small world stuff
Lots of accessible shallow shelves so things don’t get lost.
Pegboard wall for storage and display.
Ladder system shelving that can easily fit a desk that can be height adjustable (cheap and practical)
Swings/hammocks
Good lighting.
Of the ceiling was high enough then a mezzanine platform.
Easy to clean floor.
Space and stuff to make dens.
Climbing handholds on a wall.
A kind of ‘serving hatch’ window for noseying what the DC are doing and chucking food in a la zoo feeding time.
Blackboard/whiteboard wall that’s magnetic.
A system of storing dressing up crap so it’s easy to find and put away.
Lego tape that sticks on walls and makes roads.
A housekeeper to clean all this shit up at the end of the day 
We have 5DC and, even though they all had different play interests, from experience this sort of stuff is what they all liked.