as someone who’s got a close relative who happens to be a dentist, here are my recommendations based on gifts that people have actually given her.

• chocolate bar, box of chocolates - might seem a bit counter intuitive, but it’s actually the most common gift given by far. can be a hit or miss, especially if you don’t know the persons preferences / alergies / diet.
• tea or coffee - honestly a really underated gift, surprised it’s not mentioned more offten, also most doctors drink coffee so if you’re unsure about whether you should get tea / coffee, the latter is the safer choice
• small household items like coasters, hand towels, potted plants - not as commonly given but a lot more memorable, pretty much always usefull in some way, (she uses the decorative turkish towels one patient got her 6 years ago to this very day)
• something handmade - listed last because it’s by far the hardest to do, since your options are pretty limited unless you’re skilled in some sort of handiwork, the easiest to do i imagine would be some sort of caligraphy / word art or (even a relatively simple) origami project.

but of course something like a thank you note will do just fine, a little kindness and appreciation can go a long way :)