Need a cool gift for a physics guy

I hope you guys /gals won’t be upset with me for using this site to ask my question, but here goes. I had a German foreign exchange student 10 years ago. We are still in contact and I am going to visit him in August. He has received his master’s degree in physics and is now working on his PhD. I would really like to take a gift that is both functional and fun. Any and all ideas would be welcome. Thank you.