There are a number of ways through which you can come up with your own treasure hunt ideas for adults. Unlike the kids' ideas, planning a treasure hunt for adults would take more time and would also require you to know more on what the adults are into.
Whatever you decide, just make it fun and test out your treasure hunt for adults before the big day. There is nothing worst than getting halfway through a game, only to find out the organizer has forgotten something vital. A test run can be fun with friends and family, while also ironing out any issues before you start out with your main group that the hunt is for.