If you don’t mind dropping a bit–but not like a TON of money–you can buy what’s sometimes called a “pastoral stole,” which is a kind of stand-alone stole not part of a set of vestments. They often come reversible in white in violet since those are the colors used for baptisms, violet is used for confessions, and so on. They are often heavily embroidered or have a design all over them. IMO this is the cheapest functional gift worth giving to a priest. The only other ideas I have are a new rectory (lol), chalice, a set of vestments or something like that. Those things are all rather expensive.
You can look around eBay and vestment makers’ websites. Look at all the stoles.
My one warning is not to buy some random knick-knack that you would buy for just anyone, unless you know him personally and you know what he likes. I think you should buy something specifically related to priestly ministry, hence the pastoral stole, which isn’t cheap but is cheaper than most other items.