I always brought a thank you card to write out and give them on the last day, and if I spent time with a lot of techs/assistants I would also spend like $5 to pick up a container or two of cookies from the grocery store the morning of the last day.
This is totally not required but I also splurged and gave a slightly more personalized gift to two of the places I went because I really hit it off with doctors mentoring me at those two locations, and I was planning to apply for a job at both. One doctor told me he always semi-jokingly asks for a case of beer for putting up with the students and no one had ever given him one, so I got him a 6 mix pack of beers which he was super excited about. The other place, my cat wine bottle collection came up in convo at some point and one of them mentioned she really wanted one/loves that type of wine, so I got her a bottle of that, and got the other doctor a halloween themed gift (her fave holiday and it was coming up soon after my externship).
I'll add that being on the other side of things - I don't ever expect anything, but have thus far gotten thank you cards from most of my externs. One or two wrote an email thanking me afterwards instead. I think one did give me a little keychain or something, but don't entirely remember. You truly don't need to splurge on anything extra because I'm going to remember you based on how you performed during the externship and not based on whatever you bring at the end.