Air, sun and play their friends are natural. It's already a lot, but let's add something. A nice walk is always good and there is something for all tastes, suitable to satisfy even the adults: easy trails, demonstrations of plant and animal life, streams and lakes that are an irresistible lure. Even a picnic, proposed in the right day and in the right place, peppered with a few games, it's a nice attraction. Condense the desire to socialize and compete with suitable sports initiatives, mountain bikes for older children, for example, with a brisk downhill on a sled, or a treasure hunt .. and you're done.
And if they can do things the "big", why not?