This was during grad school for me. We had mandatory talks and conferences that were so annoying but we went because mandatory. However, we never went for those that weren't till...
Professor: So this Thursday, Dr xyz will be here to give a talk. As you know it is not mandatory, so if you will attend, speak up so I can write your name down
Class: dead silence.
Professor: No one?
Class: No comment.
Professor: I will be giving 2 extra marks to anyone who shows up.
Class: One or two people speak up saying they'd attend
Professor: I should probably also mention we will have pizza, sandwiches, a few desserts, cookies and some-
Class: What time? Is it okay if we come early? Do we eat before or after the talk? What kinds of pizza?
From that day onward, we had free food for every non-mandatory talk/conference and we went for them all, (sometimes even when we didn't have classes that day, we'd come from home for the conference just for the food and sometimes drinks at the bar in the school) in addition to the foodless mandatory ones lol