Big park you don't normally go to,, picnic (with a mini celebratory bottle of something special for you!), take a ball. Relax and spend the day. They get tired so will go to bed on time, you have a takeaway.
Cineworld has £1 a ticket movies at the moment. I take popcorn and drinks with me.
Museums have deals on, especially if you are on Universal Credit, can get in for £1 for a year at the London Transport Museum in London.
Go swimming! Picnic after.
Go to the woods. Walk (with bikes and scooters), play in the park, scooter.
Do a spa day at home if the kids are old enough
Take the kids to a shop with a bit of everything (my two love Home.Bargains) give them a budget to buy you a gift, reslly interesting to see what they buy you. They choose the cake.
Art galleries my kids love, what piece of.art do you like the best, which one do you dislike the most?
Treasure hunt- make a little list with pictures on of a leaf, a stick, a pinecone, an interesting stone, etc. Stick to a paperbag, kids have to find all the items for birthday cake.