Tire climb, fun for the grandkids! I also see a raised garden. Place large platters or ply wood in the bottoms, fill with potting soil or soil substitute and have at it. Would look nice with trailing plants.
Activity board (also known as busy board, sensory board, touch board, latch board etc) is an excellent fine motor development educational toy in the best principles of Montessori. This activity board (Best Christmas Presents)