Fun, free things we used to do:
Patio painting (jam jar full of water, thick paintbrush).
Hosing down the children in the garden - they loved jumping over the jets of water.
Hatching - covering them with cushions and pillows and watching them hatchling baby dinosaurs.
Drawing room camping - blanket over the back of the sofa propped up with chairs - tea on the floor - sandwiches, cake, fruit, squash.
Deer hunt and picnic in Richmond Park (v good when rainy- picnic could be had in the car or if it brightened up in the 0layground).
The museums NHM and science - the joy of living in London.
Story time at the library. Met my best grown up friend there and the DC had a ball - and squash and a biscuit - all for 20p.
Local garden centre with huge cold water fish sheds - better than the aquarium.
Keep an eye on the Moon, Mars, eclipses, pink moons, etc, and picnic under the stats in the garden.
Leaf kicking and conker collecting.
Crabbing and rock pooling on holiday.
Collecting shells, interesting stones and bits of glass at the beach.
Looking at flowers, colours and the sky.
Helping with cooking.
So many things to do that cost v v little. I loved it when dc were small.