People are always trying to get rid of mixed bags of Lego.... we have never bought any but seem to have accumulated a huge crate of it. I am sure someone in your Freecycle group will have some. Playmobil is another that gets given away. In fact anything with small sharp bits that get left on the floor for parents to stand on go for buttons.
The Works do cheap kids books. One really tight year I got the kids library cards and the promise they could get new books every 2 weeks... they were too young to realise that they are free to everyone.
"kits" - I still do these.
Printing = 2/3 bottles of poster paint, paper, interesting shapes (cut sponge, anything with a texture - leaves, plastic toys, etc) and an colouring pen to put borders or outlines on.
Paper mache = newspaper, white tissue to cover. Make paste from flour & water (1 part flour 4 parts hot water). You can use card (boxes from teh supermarket cut to templates) for structure to make 3D things. Paint & brush to finish.
Scarf / hat making kit - Chunky knitting needles or crochet hook (charity shops are fab for these). Instructions from titnernet and a ball or 2 of approriate wool.
Animal sewing kit - there are loads of templates on the internet, get some fabric (clothes from charity shop), cut out the fabric shapes, add needles, buttons cotton as required.
Explorer kit = sewing kit to make a bag to hold magnifying glass, notebook, pencil, map of local woods (or the like) and if you can find a cheap one a compass.
Bug finder kit = large jar (I get these from the recycling bins), magnifying glass, notebook, pencil and a print off of some bugs. worms etc to identify. I make these into a little book.