Just outside Newry is Slieve Gullion Forest Park. The carpark is free and they've a giant playpark, also all free. There's a cafe and ice cream vans or you can bring your own picnic.
What we had visited for however is the new sculpture walk, complete with magical fairy village. I would have gone wild for this sort of place as a child. The walk is around a mile through the forest, and again it's totally free. It's definitely work the drive down if you're from near Belfast like us.