Gift ideas for my internship?

Gifts from managers to employees are okay (somewhere I still have the pen set a boss gave me).

Gifts from employees to managers are not, but the exception is food.

For one person, if I knew they drank, I'd get a bottle of wine. For a group, food and a thank you note. That way the item is consumed. These days, a fruit basket is safest.

No one needs more "stuff".

A handwritten thank-you card to each person and something edible for the staff room would be lovely.

That depends on how much you are willing to spend but I'll just put myself in your situation and try to imagine how much an intern can spend without finishing up whatever you earned from that company. So buy something that they can use and will be a memory too. Such as a perfume or a picture frame or a nice painting of their favorite band or football team or a nice bed sheet or a nice suit.This way, you win both ways, they'll be using it too and they'll remember you too.

A joint something like edibles, fruit or snack basket, could be nice.