The TESCO vouchers suggestion is a good idea, you get much bigger savings on 'experiences' than on say shopping.
Blue Peter badges are also really good, kids [with a paying adult - but both parents don't always have to go] can get into loads of good places for free. The kids can get a badge by sending in any old crap, just a scrappy drawing will do.
I note that the kids already go to cubs, that's actually a pretty decent starter in terms of a decent amount of activities.
Camping trips are well worth considering.
I agree that special memories are important. A good way to help boost these is IMO to go places with other families, the kids will have way more fun if they're able to run around in a little pack.
e.g. during this week I took my kids to a local beauty spot, they met up with their cousins who they'd not seen in ages. we had to pay £12 to use the site's parking [with admission otherwise free]. on a stricter budget we could have parked a couple of miles away and walked, but it'd have put a fairly serious downer on things. ate a picnic. at one point the kids asked for fizzy drinks, I said no, we'd brought [tap] water bottles. at one point they asked for ice creams [something absurd like £3 each], I said yes because it was a hot day, they don't often get chance to eat that sort of thing, it'd been a couple of hours since lunch, but on a very tight budget I could have said no without very seriously impact the day out [could maybe have brought some nice sweet or chocolatey type tihngs from a supermarket]. Most of the fun came from splashing around in a river, playing with sticks and stones, etc. we took some great photos and videos, I think they'll remember the day for a long time.