Thegirloverthere- how old are your children?
Some cheap/free activities we do are:
Den building- inside and in the garden depending on the weather.
Alphabet hunts- your children have to find something beginning with each letter of the alphabet around the house/park/garden.
Ice breaking- my boys love this- I get some plastic toys- animals/dinosaurs etc and put them in bowls of water and freeze them, then I give them a normal kitchen knife/small hammer and they spend ages digging out the toys.
Baking- not too pricey and you can eat what you make! Simple biscuits are the best- we have a really quick and easy recipe.
I spy hunts- make a sheet of things they have to spot on a walk and get them to tick them off as they go.
Bug hunting/nature trails
I tend to buy a multi pack of flakes/ice cream cones and cheap vanilla ice cream- then when they complete a challenge- I spy hunt etc they can have an ice cream as a reward- but doing it yourself is much cheaper than buying them from a shop/van. I got a pack of 4 flakes for a pound (each one can be cut into 3) plus cones for a pound and ice cream for a pound- so that's 12 99' ice creams for 3 pound.
Hope some of these suggestions are useful- can probably suggest more things if I know how old your children are.