Look out for events like open farm weekend or try sailing/climbing etc at your local club, these are usually free or really cheap.
Go fishing, rock pooling, nature watching.
Mini golf/putting at local parks.
Find a local stream to paddle in.
Do you have bikes? Take them out for an adventure on those.
Go camping.
The sun newspaper (I know its an awful paper) starts it's free entry to Alton Towers token collect today.
Blackpool illuminations.
Firework competitions at various seaside resorts for free.
Lifeboat days.
Build a den in the woods.
Go to museums for free.
Go to an RAF base or airport and watch the jets take off with a picnic.
Get your name down for an allotment.
Climb a mountain (Snowdon is easiest).
Go to an impressive waterfall.
We didn't have much money but did all of the above many times and I don't think our DC missed out.