Write yourself a letter or picture , go to shop buy stamp, put stamp on adressed envelope, walk to post box, put envelope in postbox, wait for postman, open letter - big thrills all round (honest)
make little sailboats out of whatever you can find lying around - blue tack, bottle tops etc, fill a tray or platter with water, blow boats with straws and have races.
get an old argos catalogue or somesuch, choose jewellery together, cut it out and stick it on yourselves with sticky tape. My mum forgot and went out for the evening in stuck on sapphire and diamand earrings.
get a load of polystyrene packaging or beanbag beads or torn up tissue or something and make a right mess on the floor with it then get the dyson out and let ds hoover it all up so he can be just like daddy - see I've got the men in my life well trained.
Go for a numberhunt choose a number go for a walk and give ds a jellytot or something everytime they spot a number on a house, car,sign etc.
make your own playdough. 3cups plain flour, 1 and a half cups salt, 3 cups water, 1 tablespoon oil, 1 tablespoon cream of tartar (this is the secret ingredient apparantly). Mix all together then split into different saucepans (if your going to make different colours) and heat over low-medium heat with food colouring and a few drops lavender oil or somesuch to make it smell nice, stirring with wooden spoon until it all starts to come away from the sides nicely. Non stick pans work best but not essential. Very easy, Very theraputic and you'll feel nicely smug afterwards.