I've loved hiking since I was a kid. I wouldn't go for any long hikes lest they get tired and bored, but who doesn't like spending some time outside?

Aquariums and natural history museums can be a blast. If any of your kids have a specific interest in something (history, art, aviation, etc) then there may be other museums they'd like too

Unfamiliar transportation can be a freebie between activities - a boat, a train, a cable car, even a double-decker bus hit different than a car ride

Obvs, having some extra activities that you won't mind canceling can help leave time for gratifying any whims that strike the vacationers, without putting pressure on them to find something that strikes them

Landmarks can be cool, but crowded ones usually weren't worth it to me as a kid. But then, maybe your kids don't mind crowds

Swimming. Just swimming. If it's somewhere cool, then cool. If it's in a motel pool, then that can still be cool