Treasure baskets with different materials and hidden objects in them like smooth wooden curtain rings or pinecones. Closer to six months when he was sitting well with support he loves sitting in his highchaie playing with cooked pasta, jelly, ice cubes, water etc. if you don't want him to put anything in his mouth out them in sealable clear bags. At our sure start centre DS loved a mirrored wind chime thing on a stand, it was perfect height to roll on his side and play with, small maracas. Rain shakers, also those calm bottles that are filled with water, glue and glitter and sometimes small things like Lego men, he loved them and still does. The children's centre also had things like cinnamon sticks and different herbs in small containers with holes for them to smell.