Each Placement has a last day. Sometimes they are sad, because you're just settling in and getting to know everyone and then you have to leave. Sometimes they can't come soon enough.
Interns are expected to give gifts as a way of saying thank you to their resident design agency. Sweets, cakes or pastries, are well respected gifts. Other examples I have heard of are drawings, customised tea towels and plants; the latter I am assuming was some last ditch attempt to make the agency remember her well after the placement was over. As if a creative director will be looking over to the plant when wandering who to call up for a junior position. Seems a little desperate to me.
I find the whole ritual a little strange because, espescially if you are working for them for free, it is more of an opportunity in which you would expect them to say thank you to you, but never mind. Of all my placements I have rarely given a gift. I'll be honest, sometimes I think it is plain not deserved. After all, my pennies are precious and I am not going to fork out on presents for an agency who has pretty much trodden all over me for 4 weeks.
However, if you have had a good time and you do want them to think you are a nice person, it is a good idea to leave a little something behind. For me, chocolate usually does the trick and atleast you get to enjoy it too.