Packed lunches and flasks for lunch (we bought a couple of wide mouthed ones for this year that we're going to try out so we can have hot pasta and the like as well as sandwiches, soup, and crisps....!); vouchers including Tesco "double your money" ones; special offers in papers, on cereal boxes etc; making full use of any schemes you can (e.g. Blue Light if you work in that sort of work area, any AA offers, keep an eagle eye on Martin Lewis Money Saving Expert and others).
And my recent mini project has been to download as many relevant apps as possible, they often have offers on them especially when you first join.
Download a walking tour to your phone; special offers on NT and the Heritage Open Days (I think they're September?); the Gardeners' World "2 for 1" entry scheme, they publish a magazine once a year, around May to June I think, with a card in it; free events at local colleges, farms, art centres, libraries...
It's really hard work and can be very time consuming, but is one of the ways we manage to really stretch our pennies. And going on "big" days out is limited to special occasions like birthdays. It also probably does help that we live really close to some nice beaches, parks, and small woodland/nature reserves and that we don't have particularly expensive hobbies, but I honestly don't know how larger families without pots of money do it.
Mind you, I'm of the generation where a trip to the coast, fish and chips with bread and butter and a cup of tea, and building a sandcastle or two counted as an exotic day out...