Lots of messy play for self discovery
... but only as long as you love clearing up lol ,
a Shallow water tray (like a greenhouse pot plant holder/stand) fill it with water and give them some funnels, spoons and dry sand.
They'll learn how to pick up/scoop, pour and also probably how not to try eating sand again
lol
Hand painting onto a large mirror is also a good activity - and if you're worried about them eating paint try making your own gel by using cornflour or arrowroot (which is the 'proper ingredient to use !!) mixed with water - add a little food colouring and cook until it forms a thick gel.
Paint it onto their hands and get them to experiment moving them over the surface of the mirror - in a way they also 'colour themselves in as they watch what their hands are doing !!!
Edible playdough is also good - I've used a tin of ready mixed custard and rolled porridge oats kneaded together before (actually tasted quite nice
!!) have also heard of Chocolate based one .. not sure where from now ..
For the baby lots of movement stimulation with mobiles, gentle noise with rattles and lullabies ~ wind chimes can be brilliant ... and of course heaps & heaps of eye contact, smiles and hand holding/finger play
You could also perhaps make a list of 'todos for them at home .. Tooth care ~ can they bring their toothbrush over to show you .. you could draw it together and then paint / collage the picture ?
Will think on .. but hth