One of the recent innovations in scavenger hunts is to take photographs of things. These are fun, you can be much more flexible and can put together a book of memories at the end, for the honoree or you can split up the pictures amongst the attendees.
Generally, you'd get enough disposable cameras for a couple for each group. They can do such things as take a picture of someone in the group kissing the statue of the soldier in the park, or some of them making a human pyramid on a street corner downtown, etc. At the end, take them all to a one-hour photo place, get some refreshments, and then the pictures are done and hilarity ensues!