I'm an Assistant Scoutmaster and have been an Assistant Cubmaster. For both Cub Scouts and Boy Scouts I try to emphasize everyone bringing adequate gear for the activity, as thinking it through and getting the gear and experiencing the results of being prepared are valuable in so many ways. In a way that might be hard for many on this forum to imagine, some Scouts (and their parents) come to this with little practical knowledge, so that, e.g., despite your instructions to bring a quart canteen in a day pack, they show up for a day hike hand-carrying a too-small bottle of water. In more serious outings there may be mandatory gear checks, but for situations like a day hike, especially for a newcomer, it is a gentle real-life instruction to go ahead and let them get tired of carrying that water bottle. And, when they drink it all and are thirsty again, to offer some of the extra water I brought just for that purpose. So, in that spirit, here is a list of some extra gear I often take on outings:

Water
Knit hats/caps/beanies
Disposable rain ponchos
Fleece pullovers
Blanket
Moleskin for blisters

Another hint... On an overnight outing I sometimes put a chemical light stick at the outhouse and another at the campsite, so Scouts don't get lost going to the bathroom in the night (they should have a buddy anyway, but, hey, even two can get turned around at night).

Best...
Steve