A unique experience is the hardest item to come up with because you often need to think outside of the box and pull your personal connections in for this. There is really no real criteria here other than provide an experience or an item that people cannot access on their own. So, for this post, I will just be listing examples of things I have sold to get your creative juices flowing.
Examples of Unique Experiences
Character created about winning bidder in a well known author's next book
Trip to visit and tour Jay Leno's garage (with Jay Leno if the timing was right)
Principal for a Day
Trip with a professional sport's team to an out of state game which included accommodations and riding with the team on their plane to and from the game as well as an after party post game.
Lock in sleepover at a museum with a roof top movie
Premier parking space for school pickup and drop off
Batboy (bat-kid) for local professional Baseball team
Box at a professional sports game hosted by a (well-known) retired player
Timed shopping spree at a local department store
The possibilities are endless for these types of items. What I would encourage you and your team to do is to plan an entire meeting that is set aside for brainstorming this type of item. Throw out ideas with the freedom of "no idea is a bad idea" and just go with it. Some of the best auction items sound "stupid" in your head before you say it outloud and realize that it could actually happen with the connections in the room. Eventually what will happen is one member will throw out an idea, another member will know someone with the right connections and you will find yourself with a very exclusive an unique idea.