If fine, anything with water. Eg local park paddling pool with picnic, bubbles, water fights with balloons or guns, hose or watering cans, poohsticks in stream at end of suburban garden, creating a mini pond.
Picnics whatever the weather. Carpet picnics on rug indoors if wet.
Tents and camping, real in country side it back garden, make your own for littlies inside with rug and chairs or outside with washing line and chairs.
Buried treasure. Go on a country walk and bury the treasure. Next day go and discover it.
Cardboard boxes. Huge fridge ones from curries, smaller ones. Cut a few holes in to look through. A tardis, a tortoise, a rocket, a castle.
Reading and story telling for wet days. And kim's game for memory training. Jacks, marbles, hoola hoops, quoits, pea shooting and petanque as they get older.
Card games, pairs (pelmanism) and snap,then beggar my neighbour, then racing demon and whist, dummy and ending up with black jack and poker for the teens. Pit for all the family.
Making telephones out of tin cans and string.
Teaching them to whittle.
Cooking. Not just biscuits but making pasta and pizza and potato salad and scrambled eggs.
Tree climbing if decent trees around
Plant a garden. Radishes and lettuce.
Hold a table sale outside the house sharing proceeds with fave charity.
Writing postcards to granny.
Duvet day. Cinema as above, ie dvd closed curtains and popcorn, making pizza.
Going fishing or to local pond with fishing nets and jam jars. Take picnic.
English heritage membership, children free.
Discover if your local area has free deals. Eg the local library or swimming pool. In London children are free on buses. So go on the top of the no 11 bus from Liverpool st station down the kings road and you see all the sights of London, or nearly, for the cost of one adult bus ride.