You can heighten the experience of your treasure hunt by tantalising the participants with little tastes of what is to come but without letting them know what they will be doing.
Issue tokens to use on the day for people who manage to solve little puzzles alluding to the event i.e. anagrams to work out where you will be going in exchange for a drinks voucher.
The more mystery there is, the more they will get out of it. Leave loads of gaps for their imaginations to fill in and they will fill them with more wonderful things than you could conjure up in reality.
Start off the day by having a blank envelope with some top secret instructions anonymously passed to you.
If you would like something to get people excited – you could try using www.treasurehuntdesign.com.