Go to a crowded market with lots of tourists etc, or a mall/shopping complex. Have a number of Wallys (the group leaders) in various disguise wandering around, and kids in teams have to find them. One or more leaders could dress up as Wally (from the puzzles) and be worth bonus points. This lasts two hours easily at our local market – (kids can look at the stalls as well on the way).
This can be varied in a number of ways, include scavenger hunts, cryptic clues to get to the market etc, etc.
Our last one was renamed “Mission Impossible” and included a cryptic route to the market and then to find the leaders in disguise.