We have a regular local church-run playgroup that is a £5 a go for me, DD, and DS. 1.5hrs of play, + drinks and snacks for us all, + craft activity. We walk there and back so no travel costs either.
Parks - wrap up warm. Take some old bread etc to feed ducks/birds. Monkey nuts are cheap, we buy big packs from supermarket sometimes and go and feed the squirrels in the parks. Sometimes take bubbles, or balls, or frisbee. Playgrounds. Walk around walled gardens or lakes. Sometimes take DD's scooter or balance bike. Rotate parks, we are lucky that there are 3 within easy walking distance, one has a sandpit so we take bucket and spade etc. Plenty of others are short drives away so don't cost much in fuel. One has a skate park which DD loves to use on her balance bike.
At home - films for DD, we have Netflix (shared account with relatives who pay for it) and a friend lets us use their Disney+ account! Bubble baths - I put both kids in together and they love it (4 and nearly 1).
With eldest - baking, even the really cheap easy kits, or buying pizza bases and toppings and making pizzas for tea. She helps make lunch too if it's sandwiches and I get her to help with chores like laying and tidying away the table at meal times, putting her clean laundry away, she likes to help brush up the floors, weed the front and yard, mop the floors/clean it with wipes on a pole thing, dust etc.
Play dates at your house or the houses of kid's friends/visiting relatives or having them visit.
Local library.
Popping the local supermarket or post office - to do things you'd need to do anyway but save it for when they're with you to get them out and learning about the world around them.
Swimming at a local pool, kids under 8 are free here and adults are inexpensive.
Video calls with friends/relatives.
Arty/crafty stuff at home. Magic sand kills a lot of time for us - DD will play with it for quite awhile and then also helps tidy it up which also takes ages!