That's a tough topic for my unit! We're part of the School of Sustainability at our university, so the first step is that we should not add items into the world until we have a very good reason for it. The first R is Reduce after all! (the others are Reuse and Recycle, in that order).
When we do need a gift, usually to thank a speaker rather than a blanket tchochke for all attendee, we try to source items made of recylced materials. It's a bit priceier but worth it. We work with our swag vendors to source items as locally as possible, and have items travel as little as possible.
To answer the prompt, it's a basic but a great water bottle or a high-quality tote bag are items I am likely to keep using. As for something absolutely unique we're doing right now, I'm working with our campus's arborist and carpentry shops to get slices of trees that fell on campus cut to make blank coasters, and I go to our campus makerspace to laser-engrave them for custom gifts. Can't do more personnal, unique, or sustainable than that!
------------------------------
Béline Falzon, CMP
Assistant Director for Events
Stanford University, Precourt Institute for Energy
------------------------------