Go pond dipping, or fishing.
Collect and press flowers.
Download a list of British birds (with pictures) and go and tick off how many you see on your nature ramble. Could make binoculars before you go if your kids are tiny.
Baking, ?gingerbread men or fairy cakes, something that needs decorating later to prolong the activity.
Make a video, mine like acting out Britain's got talent and naff adverts.
Pamper day, bath with candles, pedicure, manicure, lie still for hours with cucumber on their eyes, give them a massage.
Go to library and see what's on for free, then borrow a pile of books while you are there.
Find some good trees to climb.
Make a teddy from scraps of fabric.
Take the government up on it's free swimming scheme.
Go to RAF Cosford.
Let kids paint the garage door.
Have a little campfire in the garden, toast marshmallows.
Make a shop with all the packets and tins from the kitchen or even better make it into a shoe shop.