Treasure hunts
A treasure hunt involve drivers participating in a timed event would be illegal in the UK, and probably in many other countries!
The drivers may well end up in court and will have problems with their insurance policies in the future. Note that their car insurance will be invalid if they are deemed to be racing!
You may also get into legal trouble for organising an illegal event!!!
The event could be a treasure hunt or a scavanger hunt. Wikipedia defines a scavanger hunt as follows:
A scavenger hunt is a game in which individuals or teams seek to find a number of specific items, or perform tasks, as given in a list. The goal is either to complete the list first, or to complete the list in the most creative manner.
A treasure hunt is where people follow clues to eventually arrive at the 'hidden' treasure.
You can hold the event in a house and/or garden. Or perhaps in a town or village (you need to watch that the clues don't disappear!).
Sell tickets to allow people to participate. You must never turn a hunt using cars into a timed event or anything that would mean that people were racing each other or speeding.