I am not sure what kind of budget you have, and obviously your post is really outdated, but I would suggest Helicopter rides!!!
I went on one a few years back with a group of people, and it was really fun.
The other fun thing would be a bus tour over to the Hoover Dam - http://hooverdamtourcompany.com/
I would do something fun and different each year.
When I take my clients to the Philippines every year, I always do my favorite, and then I think of a couple other things and let everyone pick one as a group.