Hot flask, binoculars, waterproof jacket, lunch...all essential for a walk in the backcountry, true but many a lunchtime break has been damped by rain and soggy, soggy sandwiches.
Carrying a tarp in your rucksack when outdoors has become a habit for me, being from the island paradise that is Great Britain has meant I've had to grow up with a constant watch on the horizon for rain even in the summer, and a good lightweight tarp in my pack has saved many a lunchtime from an otherwise miserable fate.
Often seen as a roof for camping overnight, the tarp can be a more regular feature to your kit list. With practice a person alone can erect a vital parapet against the elements in under a minute, something I teach on our bushcraft courses; and with a couple of minutes, or the help of a friend or two, a lofted canopy can arranged to sit under.
These simple techniques can provide with very little effort a shelter for adults and children alike whilst out braving the weather. I'd advocate strongly going for a hike on rainy days: you'll get the woods to yourself, and feel the swirling vortex of Nature pulling you off on your own private adventure. But of course a shelter can be a welcome 'home' even when the weather is fair: be it a sun-shade or a fixed point in the wilderness and a marker far seen where packs can be left whilst toilet breaks or animal watching might take us a few meters in all directions. My children love a tarp-base when we're out in countryside, even if only for a half hour, it provides them with a touchstone from which they can wander and explore by themselves whilst keeping 'home' in sight.
I generally carry a lightweight, 3x3 metre tarp with me, it can provide shelter for between 6-8 people to stand underneath, and a square tarp is more versatile in the arrangements you can use to set it up. A modern tarp can also be packed very small into a dry-sack, taking up a negligible amount of space in your rucksack and weighing almost nothing. I recommend using an oversized dry-sack to pack it into for a couple of reasons: importantly this will stop your potentially very wet tarp soaking your other kit when you pack it away; and secondly having a dry-sack bigger than you need means it's very easy to pack the tarp away into it when you break camp. A little tip: pack it tightly into the dry-bag but leave it a little squashy, it will stuff better into the gaps between your kit in your rucksack than it would if you stuffed it into a hard ball.