Thank you all for your ideas, here is a quick summary of all of them so far, that I collated from your comments, as well as a few that I found also. I'll create another summary if anymore get added this week.
BIRD FEEDERS - Make bird feeders using pots or recycling bottles.
CARDBOARD CASTLE - Collect loads of cardboard and 1 day can be filled with creating a cardboard castle. You just need some very strong tape, string for a drawbridge and some scissors
CHORES BINGO - Gain points for doing household chores. Make the chores funny like 'put away the washing walking like a chicken' if they need extra motivation.
CINEMA DAY - Have fun crafting tickets and movie posters, have a choice of 3 films, microwave popcorn other snacks etc put blankets up at the windows etc.
DRAWING CHALLENGE - Get your kids to draw your family members or pets and get a cheap frame from Ikea and showcase their talents in the hallway
DENS - Make a den inside the house with chairs and blankets. The perfect place to spend the days with friends and having lunch
PIZZA/BAKING - Make a pizza or general food preparation for a meal. Create a shopping list if necessary and go shopping for ingredients
PARCOUR - Build a parcour with chairs and pillows (don't touch the floor/only use feet - no hands for balance)
TREASURE HUNT - Treasure hunts in the home where one clue leads to another then the prize at the end
THEME DAY - Make decorations for a disco at home. Have a theme based on a country e.g. Italian, Mexican. Design the flags, make that cuisine to eat, learn a bit about the country or a few phrases to say
TALENT SHOW - Have a talent show. Each child practises their skill to show e.g. gymnastics, football, write a play. This could be an end of week thing they practise towards.
LIBRARY - Free activities at local libraries like reading challenges, Lego groups, coding clubs etc
GEOCACHING - Found a website about this, looks like it could be fun doing this massive treasure hunt - Both are cheap / free. You can explore new places and as an extra bonus the children get lots of exercise.
HOLIDAY CLUBS - Churches often have free/cheap holiday clubs eg 10-12 for a week. Some might do sports ones too.
WILDLIFE TRUST - The local Wildlife Trusts to do free kids sessions, they're all focused around local parks and wildlife spots. They are doing bug hunting, den building, making bug hotels etc.
SCIENCE EXPERIMENTS - These can be good fun. Lots of ideas here but there are lots of other sites
MINATURE GARDEN - Create a miniature garden in a box or foil tray. Use soil and grass seed and put a few flower seeds in there and see what you can come up with!
GARDEN CAMPING - If you own a tent, put it on the back garden on a nice summers night and make S'mores and memories with the little ones.
SOCK BUBBLES - You cut the end off the plastic bottle and put a sock over the cut off end. Dip the sock end of the bottle in bubble mixture and blow into the neck of the bottle. Loads of foamy bubbles should come out.
JUNIOR PARK RUN - Junior park runs are free and aimed at 4-14 year olds.
MAKEOVERS - They could give you a makeover. They can make face masks out of porridge oats and water and put it on you with cucumber for your eyes. Then a foot spa with bubble bath and water in a basin. They could do your nails.
FROZEN T-SHIRT CHALLENGE - You could do a frozen t-shirt challenge. You get an old t-shirt, soak it in water, roll it up tight and fold it etc. Then freeze it overnight. The game is to be the first one, or by X amount time, to be wearing the shirt!
MARBLE RUN - Make a marble run from loo rolls and a cardboard box or two.
ROCK PAINTING - Make a rock snake or ladybird or other creature for the garden or leaving them in woodlands for others to find.
QUIZZES - A great resource here for kids quizzes. Quite a few different ones including loads of Disney ones!