Sometimes people want to organise a secret or surprise event. This can be fun but you need to take care over the type of event that you organise. A Treasure Hunt, for example, will not be a problem for the majority of people. You do need to make sure that people don’t have any recent or recurring injuries if they are walking any distance, but apart from this it is one of the least problematic events.
However, organising a secret high ropes course event would a different matter. People do need to be given warning and an opportunity to consider whether they want to participate and, if necessary, to discretely exclude themselves.
The problem with organising a secret event is that people will not know what it is going to be, and many will fear the worst. This may mean that you have people excluding themselves (weddings and illnesses, whether real or not) and a higher rate of no shows on the day.
It's not an easy thing to say strictly yes or no on, you will know your group better than anyone and whether this will be popular. However, the critical advice is that if the activity involves any risk at all, or strays into the realms of potential phobias, it is very risky to withhold information from the people who will be participating.
Some examples of activities that would need judgment are those involving heights, water, driving, shooting and animals. They make seem like a pretty comprehensive list, but there are still lots of activities that can be safely kept secret, for example treasure hunts, team challenge activities and film making.