Best way to answer this question is to assess the event that you are attending/hosting.
What is the event for? Who is the event for?
I attend multiple events as a representative for a University. At the hospital events, the attendees love to get pens, notepads, hand sanitizer, stress balls, bags and a few more items. Different people thought out of the box by offering cold/hot packs and another had a fun stress ball item that looked like a duck with scrubs on.
My suggestion is to find out who you are attempting to present to and go from there. Usually it might take an event or two like it to give you a feel of what goes out the most. Pens are always a must have, they are easy to distribute and everyone likes them.
If you can be as out of the box thinking as possible to make it unique, the better. More people are attracted to the item that nobody else has. Depending on the event, it could be a number of items.
Hope that information helps.